Out of curiosity, what's the field? In physics nothing is particularly open-access but that doesn't matter because everything with any significance at all gets put up on arxiv six months before publication.
Well, it does matter somewhat: authors still publish in journals at the traditional publishers, and universities still subscribe to it, wasting research funds in developed countries and making the barriers to participation higher for researchers in emerging economies.
For its conferences, ACM SIGPLAN asks the authors to pay for the open-access fee if possible, but will cover it if the author cannot afford it.
Don't know about OP. In my experience a lot/most Computational Biology papers are Open Access.
I can second this. I've had a much better experience in Computational Biology compared to other fields wrt paper access.