whats the difference between an open api and a protocol?

An open API can implement a protocol yet a protocol is different from an API because it exists as a specification for communication or actions.

An open API is also typically owned by the entity running it while a protocol doesn't necessarily need to be run in the same way depending on what it defines.

An example: HTTP as a protocol describes how we all communicate and AWS S3 uses HTTP to function but as an API is not a protocol itself.

Great example with S3!

There are multiple S3-compatible offerings from viable competitors (e.g., BackBlaze, Cloudflare) as well as battle-tested self-storage options.

Amazon can offer new functionality within S3; others cannot and must follow.

S3 is not a protocol. It becomes a protocol with effective and credible governance, despite the existence of consensus and running code