diff --git a/.changeset/eighty-rivers-enjoy.md b/.changeset/eighty-rivers-enjoy.md
new file mode 100644
index 000000000..c7b208876
--- /dev/null
+++ b/.changeset/eighty-rivers-enjoy.md
@@ -0,0 +1,5 @@
+---
+"@hey-api/openapi-ts": patch
+---
+
+fix: make descriminator field required when used with `oneOf` keyword
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-mapped-many/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-mapped-many/types.gen.ts
index 1f6c5f2f2..e48cc7825 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-mapped-many/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-mapped-many/types.gen.ts
@@ -1,9 +1,9 @@
 // This file is auto-generated by @hey-api/openapi-ts
 
 export type Foo = ({
-    foo?: 'one' | 'two';
+    foo: 'one' | 'two';
 } & Bar) | ({
-    foo?: 'three';
+    foo: 'three';
 } & Baz);
 
 export type Bar = {
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-one-of/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-one-of/types.gen.ts
index cdd9c641f..c739a4d95 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-one-of/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/discriminator-one-of/types.gen.ts
@@ -1,9 +1,9 @@
 // This file is auto-generated by @hey-api/openapi-ts
 
 export type Foo = ({
-    type?: 'Bar';
+    type: 'Bar';
 } & Bar) | ({
-    type?: 'Baz';
+    type: 'Baz';
 } & Baz);
 
 export type Baz = Qux;
@@ -18,9 +18,9 @@ export type Qux = {
 export type Quux = 'Bar' | 'Baz';
 
 export type Quuz = ({
-    type?: 'bar';
+    type: 'bar';
 } & Bar) | ({
-    type?: 'baz';
+    type: 'baz';
 } & Baz);
 
 export type ClientOptions = {
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/default/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/default/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/default/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/default/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/throwOnError/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/throwOnError/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/throwOnError/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@hey-api/sdk/throwOnError/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/asClass/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/asClass/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/asClass/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/asClass/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/types.gen.ts
index a152624a2..170641a8a 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/fetch/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/fetch/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/fetch/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/fetch/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/asClass/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/asClass/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/asClass/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/asClass/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/types.gen.ts
index a152624a2..170641a8a 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/fetch/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/fetch/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/fetch/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/fetch/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/asClass/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/asClass/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/asClass/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/asClass/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/types.gen.ts
index a152624a2..170641a8a 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/fetch/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/fetch/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/fetch/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/fetch/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/asClass/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/asClass/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/asClass/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/asClass/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/types.gen.ts
index a152624a2..170641a8a 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/fetch/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/fetch/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/fetch/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/fetch/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/asClass/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/asClass/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/asClass/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/asClass/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/types.gen.ts
index a152624a2..170641a8a 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/fetch/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/fetch/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/fetch/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/fetch/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/fastify/default/types.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/fastify/default/types.gen.ts
index 4f2e24b56..c2e9e8e23 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/fastify/default/types.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/fastify/default/types.gen.ts
@@ -442,9 +442,9 @@ export type ModelSquare = {
  * This is a model with one property with a 'one of' relationship where the options are not $ref
  */
 export type CompositionWithOneOfDiscriminator = ({
-    kind?: 'circle';
+    kind: 'circle';
 } & ModelCircle) | ({
-    kind?: 'square';
+    kind: 'square';
 } & ModelSquare);
 
 /**
diff --git a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/zod/default/zod.gen.ts b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/zod/default/zod.gen.ts
index c4e3b9123..ebd85ab94 100644
--- a/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/zod/default/zod.gen.ts
+++ b/packages/openapi-ts-tests/test/__snapshots__/3.0.x/plugins/zod/default/zod.gen.ts
@@ -274,10 +274,10 @@ export const zModelSquare = z.object({
 
 export const zCompositionWithOneOfDiscriminator = z.union([
     z.object({
-        kind: z.literal('circle').optional()
+        kind: z.literal('circle')
     }).and(zModelCircle),
     z.object({
-        kind: z.literal('square').optional()
+        kind: z.literal('square')
     }).and(zModelSquare)
 ]);
 
diff --git a/packages/openapi-ts/src/openApi/3.0.x/parser/schema.ts b/packages/openapi-ts/src/openApi/3.0.x/parser/schema.ts
index 15b5b4bf9..f43caaf3e 100644
--- a/packages/openapi-ts/src/openApi/3.0.x/parser/schema.ts
+++ b/packages/openapi-ts/src/openApi/3.0.x/parser/schema.ts
@@ -710,6 +710,7 @@ const parseOneOf = ({
                 }
               : valueSchemas[0]!,
         },
+        required: [schema.discriminator.propertyName],
         type: 'object',
       };
       irCompositionSchema = {
diff --git a/packages/openapi-ts/src/openApi/3.1.x/parser/schema.ts b/packages/openapi-ts/src/openApi/3.1.x/parser/schema.ts
index 315fceb63..a5d2405c5 100644
--- a/packages/openapi-ts/src/openApi/3.1.x/parser/schema.ts
+++ b/packages/openapi-ts/src/openApi/3.1.x/parser/schema.ts
@@ -423,6 +423,7 @@ const parseAllOf = ({
                   }
                 : valueSchemas[0]!,
           },
+          required: [ref.discriminator.propertyName],
           type: 'object',
         };
         if (ref.required?.includes(ref.discriminator.propertyName)) {