I have no direct experience with Stanford OHS, but my friend's DD has taken some courses. The course format is a virtual classroom, with scheduled class times and lectures you attend by videoconference. There is the usual homework, quizzes, do-at-home but reasonably rigorous labs where appropriate, etc. The instructor holds office hours online.
I have heard praise and complaints about course quality- apparently it's not very consistent, but the good ones are very very good.
I believe they accept anyone ready for the work, and you can attend anything from one class to a full HS enrollment. They are rather expensive. I have no idea whether they do financial aid, but it never hurts to ask.
Hopefully someone on this board will have more information than I can give you!
