In theory, it should be easier to do for OSC, because NMS was downloadable content - the song files for that song are stored on your 360/PS3's hard drive. OSC is a stock song in the game itself, so the information for it is stored on the Rock Band 2 disc. I know there's a program for ripping songs from the Guitar Hero game discs (Guitar Hero Explorer a.k.a. "GHex"), but I have no idea if a variation of it exists for the Rock Band series.