Earlier quoted context omitted.
We're talking in English, not in Go. The meaning doesn't change that much because of using uppercase initials. What you're referring to has already been consolidated as "source available".
I'm not a native speaker, but to me "open" sounds like it fits to the case when I can see the code. Am I speaking in Go?
As others said, while "open" does indeed mean "reachable" or "available" in this context of source code, it happens that "open source" is a well defined thing to allow not only access, but also modification, reuse, and distribution without limitations. So the "open" in "open source" has its meaning brought to the highest level of openness.