NativeJovian

What sort of legal issues have you faced for posting music based on copyrighted material? Have you gotten cease & desist orders or been sued by a copyright holder?

You mention filing for 501(c)3 status and becoming an official non-profit group. Why is that important? Does it grant you any legal protections other than tax benefits? Are there any drawbacks/risks involved with nonprofit status?

Depends on your definition of "famous", but several professional musicians, including VGM composers, have posted remixes on OCR. Jeremy Soule (composer for the Elder Scrolls series since Morrowind, among other things) and Hyadain (aka Kenichi Maeyamada, composer of various anime and J-pop songs) are two off the top of my head, not even counting long-time contributors who have been posting on OCR since before they've done professional work

I'm pretty sure I found a random OC ReMix on Napster or one of those file-sharing programs (remember those?) back in like 2000-2001-ish. I either found the URL in the tags or just searched "oc remix" to find the main site.

Can't remember which actual remix it was, though.

You can actually see which songs have been remixed most by going to the song list and ordering by number of mixes. Terra's Theme from FFVI is, in fact, in the top spot.