This document discusses design-first and code-first approaches to API development. It explores how existing services can leverage the OpenAPI Specification (OAS) and the benefits of each approach. Design-first allows for a single source of truth across design, development, testing and documentation. It enables early feedback and iteration. Code-first treats OAS as a byproduct of development and enables existing practices, but requires more customization. The document provides examples of how teams have implemented both approaches using SmartBear tools.