diff --git a/src/models/index.ts b/src/models/index.ts
index 6e2626a..11c7316 100644
--- a/src/models/index.ts
+++ b/src/models/index.ts
@@ -1,2 +1,3 @@
 export { AnthropicClient, type AnthropicClientConfig } from './anthropic.js';
+export { OpenAIClient, type OpenAIClientConfig } from './openai.js';
 export type { Message, ChatRequest, ChatResponse, ModelClient } from './types.js';
diff --git a/src/models/openai.test.ts b/src/models/openai.test.ts
new file mode 100644
index 0000000..1b37a0d
--- /dev/null
+++ b/src/models/openai.test.ts
@@ -0,0 +1,33 @@
+import { describe, it, expect, vi } from 'vitest';
+import { OpenAIClient } from './openai.js';
+
+vi.mock('openai', () => ({
+  default: vi.fn().mockImplementation(() => ({
+    chat: {
+      completions: {
+        create: vi.fn().mockResolvedValue({
+          choices: [{ message: { content: 'Hello from GPT!' }, finish_reason: 'stop' }],
+          usage: { prompt_tokens: 10, completion_tokens: 5 },
+        }),
+      },
+    },
+  })),
+}));
+
+describe('OpenAIClient', () => {
+  it('sends messages and returns response', async () => {
+    const client = new OpenAIClient({
+      apiKey: 'test-key',
+      model: 'gpt-4o',
+    });
+
+    const response = await client.chat({
+      messages: [{ role: 'user', content: 'Hello' }],
+    });
+
+    expect(response.content).toBe('Hello from GPT!');
+    expect(response.stopReason).toBe('stop');
+    expect(response.usage.inputTokens).toBe(10);
+    expect(response.usage.outputTokens).toBe(5);
+  });
+});
diff --git a/src/models/openai.ts b/src/models/openai.ts
new file mode 100644
index 0000000..3b75ca0
--- /dev/null
+++ b/src/models/openai.ts
@@ -0,0 +1,54 @@
+import OpenAI from 'openai';
+import type { ChatRequest, ChatResponse, ModelClient } from './types.js';
+
+export interface OpenAIClientConfig {
+  apiKey?: string;
+  model: string;
+  maxTokens?: number;
+  baseURL?: string;
+}
+
+export class OpenAIClient implements ModelClient {
+  private client: OpenAI;
+  private model: string;
+  private defaultMaxTokens: number;
+
+  constructor(config: OpenAIClientConfig) {
+    this.client = new OpenAI({
+      apiKey: config.apiKey,
+      baseURL: config.baseURL,
+    });
+    this.model = config.model;
+    this.defaultMaxTokens = config.maxTokens ?? 4096;
+  }
+
+  async chat(request: ChatRequest): Promise<ChatResponse> {
+    const messages: OpenAI.ChatCompletionMessageParam[] = [];
+
+    if (request.system) {
+      messages.push({ role: 'system', content: request.system });
+    }
+
+    for (const msg of request.messages) {
+      messages.push({ role: msg.role, content: msg.content });
+    }
+
+    const response = await this.client.chat.completions.create({
+      model: this.model,
+      max_tokens: request.maxTokens ?? this.defaultMaxTokens,
+      messages,
+    });
+
+    const choice = response.choices[0];
+    const content = choice?.message?.content ?? '';
+
+    return {
+      content,
+      stopReason: choice?.finish_reason ?? 'stop',
+      usage: {
+        inputTokens: response.usage?.prompt_tokens ?? 0,
+        outputTokens: response.usage?.completion_tokens ?? 0,
+      },
+    };
+  }
+}