The Bunq Oauth Client API is a service that allows developers to easily integrate the Bunq banking platform into their applications. By utilizing OAuth authentication, users can securely connect their bank accounts to third-party apps for seamless access to their financial information. The API provides endpoints for managing user sessions, retrieving account information, making payments, and more. With the Bunq Oauth Client API, developers can create innovative financial tools and services that enhance the banking experience for users.