I don't put much stock in OPR because I know that I am a very average tourney player and on Full Tilt it has me in the 93 or 94 percentile (actually that's last 120 days, I'm not sure about "full search", and site is down so can't check). I have no idea how they figure out thier ranking to be quite honest with you. I'll take it, but I know there are much much better players then me ranked below me on Full Tilt.
I would think that ITM, ROI, and total profit would have to be put together somehow to figure out the rankings, but I can't really find any method to the madness. You would also have to think that they should figure the size of the field in a tourney and an avg. finish somehow into the mix.
If you can figure it out though, I would love to know.