diff --git a/common/changes/@typespec/openapi/openApi-FixNamespace_2023-09-19-19-48.json b/common/changes/@typespec/openapi/openApi-FixNamespace_2023-09-19-19-48.json
new file mode 100644
index 00000000000..10d05bbee43
--- /dev/null
+++ b/common/changes/@typespec/openapi/openApi-FixNamespace_2023-09-19-19-48.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/openapi",
+      "comment": "Changed namespace from `OpenAPI` to `TypeSpec.OpenAPI`.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/openapi"
+}
\ No newline at end of file
diff --git a/packages/openapi/lib/decorators.tsp b/packages/openapi/lib/decorators.tsp
index 53dde05e333..99675e33665 100644
--- a/packages/openapi/lib/decorators.tsp
+++ b/packages/openapi/lib/decorators.tsp
@@ -1,6 +1,6 @@
 using TypeSpec.Reflection;
 
-namespace OpenAPI;
+namespace TypeSpec.OpenAPI;
 
 /**
  * Specify the OpenAPI `operationId` property for this operation.
diff --git a/packages/openapi/src/decorators.ts b/packages/openapi/src/decorators.ts
index c5f100e7e9e..0c6b4f2e33c 100644
--- a/packages/openapi/src/decorators.ts
+++ b/packages/openapi/src/decorators.ts
@@ -12,7 +12,7 @@ import { setStatusCode } from "@typespec/http";
 import { createStateSymbol, reportDiagnostic } from "./lib.js";
 import { AdditionalInfo, ExtensionKey } from "./types.js";
 
-export const namespace = "OpenAPI";
+export const namespace = "TypeSpec.OpenAPI";
 
 const operationIdsKey = createStateSymbol("operationIds");
 /**
diff --git a/packages/openapi/test/decorators.test.ts b/packages/openapi/test/decorators.test.ts
index d6b608b92b3..dc604f58ab6 100644
--- a/packages/openapi/test/decorators.test.ts
+++ b/packages/openapi/test/decorators.test.ts
@@ -169,7 +169,8 @@ describe("openapi: decorators", () => {
 
       expectDiagnostics(diagnostics, {
         code: "invalid-argument",
-        message: "Argument '123' is not assignable to parameter of type 'OpenAPI.AdditionalInfo'",
+        message:
+          "Argument '123' is not assignable to parameter of type 'TypeSpec.OpenAPI.AdditionalInfo'",
       });
     });
 
diff --git a/packages/openapi/test/test-host.ts b/packages/openapi/test/test-host.ts
index 60166f3ad71..c32f07e16bb 100644
--- a/packages/openapi/test/test-host.ts
+++ b/packages/openapi/test/test-host.ts
@@ -10,5 +10,5 @@ export async function createOpenAPITestHost() {
 }
 export async function createOpenAPITestRunner() {
   const host = await createOpenAPITestHost();
-  return createTestWrapper(host, { autoUsings: ["OpenAPI"] });
+  return createTestWrapper(host, { autoUsings: ["TypeSpec.OpenAPI"] });
 }
diff --git a/packages/openapi3/test/test-host.ts b/packages/openapi3/test/test-host.ts
index cb39e3004eb..7d3c64706b1 100644
--- a/packages/openapi3/test/test-host.ts
+++ b/packages/openapi3/test/test-host.ts
@@ -36,7 +36,7 @@ export async function createOpenAPITestRunner({
   ${withVersioning ? `import "@typespec/versioning"` : ""};
   using TypeSpec.Rest;
   using TypeSpec.Http;
-  using OpenAPI;
+  using TypeSpec.OpenAPI;
   ${withVersioning ? "using TypeSpec.Versioning;" : ""}
 `;
   return createTestWrapper(host, {
@@ -67,7 +67,7 @@ export async function openApiFor(
     "./main.tsp",
     `import "@typespec/http"; import "@typespec/rest"; import "@typespec/openapi"; import "@typespec/openapi3"; ${
       versions ? `import "@typespec/versioning"; using TypeSpec.Versioning;` : ""
-    }using TypeSpec.Rest;using TypeSpec.Http;using OpenAPI;${code}`
+    }using TypeSpec.Rest;using TypeSpec.Http;using TypeSpec.OpenAPI;${code}`
   );
   const diagnostics = await host.diagnose("./main.tsp", {
     noEmit: false,
diff --git a/packages/openapi3/test/versioning.test.ts b/packages/openapi3/test/versioning.test.ts
index d05e7b3d6a7..985e6eaaf38 100644
--- a/packages/openapi3/test/versioning.test.ts
+++ b/packages/openapi3/test/versioning.test.ts
@@ -118,7 +118,7 @@ describe("openapi3: versioning", () => {
 
     const runner = createTestWrapper(host, {
       autoImports: [...host.libraries.map((x) => x.name), "./test.js"],
-      autoUsings: ["TypeSpec.Rest", "TypeSpec.Http", "OpenAPI", "TypeSpec.Versioning"],
+      autoUsings: ["TypeSpec.Rest", "TypeSpec.Http", "TypeSpec.OpenAPI", "TypeSpec.Versioning"],
       compilerOptions: { emit: ["@typespec/openapi3"] },
     });
 
diff --git a/packages/playground-website/samples/unions.tsp b/packages/playground-website/samples/unions.tsp
index 83ba04f26fa..6a6746d1afe 100644
--- a/packages/playground-website/samples/unions.tsp
+++ b/packages/playground-website/samples/unions.tsp
@@ -8,7 +8,7 @@ import "@typespec/openapi3";
 namespace DemoService;
 using TypeSpec.Rest;
 using TypeSpec.Http;
-using OpenAPI;
+using TypeSpec.OpenAPI;
 
 model WidgetBase {
   @key id: string;
diff --git a/packages/samples/specs/openapi-extensions/openapi-extensions.tsp b/packages/samples/specs/openapi-extensions/openapi-extensions.tsp
index 0cde37d762c..2e466d3c49c 100644
--- a/packages/samples/specs/openapi-extensions/openapi-extensions.tsp
+++ b/packages/samples/specs/openapi-extensions/openapi-extensions.tsp
@@ -1,7 +1,7 @@
 import "@typespec/rest";
 import "@typespec/openapi";
 
-using OpenAPI;
+using TypeSpec.OpenAPI;
 
 model MyConfig {
   name: "Foo";
diff --git a/packages/samples/specs/testserver/body-boolean/body-boolean.tsp b/packages/samples/specs/testserver/body-boolean/body-boolean.tsp
index 8646759410b..5a5f809b580 100644
--- a/packages/samples/specs/testserver/body-boolean/body-boolean.tsp
+++ b/packages/samples/specs/testserver/body-boolean/body-boolean.tsp
@@ -2,7 +2,7 @@ import "@typespec/rest";
 import "@typespec/openapi";
 
 using TypeSpec.Http;
-using OpenAPI;
+using TypeSpec.OpenAPI;
 
 @service({
   title: "sample",
diff --git a/packages/samples/specs/testserver/media-types/media-types.tsp b/packages/samples/specs/testserver/media-types/media-types.tsp
index c3fa1c72357..40b9745c950 100644
--- a/packages/samples/specs/testserver/media-types/media-types.tsp
+++ b/packages/samples/specs/testserver/media-types/media-types.tsp
@@ -2,7 +2,7 @@ import "@typespec/rest";
 import "@typespec/openapi";
 
 using TypeSpec.Http;
-using OpenAPI;
+using TypeSpec.OpenAPI;
 
 @service({
   title: "sample",