They answer different questions and people waste a lot of energy treating them as competitors.
OpenAPI is the static truth: the complete surface area of the API, every operation, every schema, every response, described precisely and neutrally. It is the artifact governance lints, SDKs generate from, and gateways configure from. There should be exactly one, and it should be authoritative.
A collection — Postman, Bruno, Insomnia, OpenCollection — is a portable, executable bundle of real requests with the environment needed to run them. It captures how the API is actually used: real auth, real values, real sequencing, in the order a human or a script would do it. Where OpenAPI describes what is possible, the collection is the contract in motion.
That is why you want several collections from one OpenAPI, each shaped around a specific outcome: a getting-started collection, a collection per major workflow, a collection for the partner integration. One source of truth, many derivatives designed for particular business outcomes.
For onboarding specifically, the collection is usually the more valuable artifact. A developer who can fork a collection, drop in a key, and get a real 200 back in two minutes is a developer you have onboarded. A developer staring at a reference document is one who might.
