If you really enjoyed O.S.I. (GREAT album) you are in luck as the Prog/Metal genre has really "taken off" during the past 10 years. I would recommend you start by listening to some Dream Theatre, Chroma Key, and Fates Warning (since O.S.I. is comprised of members from each of these bands). Dream Theatre shows up all the time in these threads, and everyone has their favorite album, but to really hear what this band is "about" I would start with "Awake" and "Octavarium". Chroma Key has only one album, "Graveyard Mountain Home", and it is exceptional. Fates Warning has done many great albums, I would start with "Pleasant Shade of Grey" and "Disconnected".
I would also recommend Transatlantic "Bridge Across Forever" and "SMPTe". These are phenominal Prog records from another "supergroup": Roine Stolt (Flower Kings), Pete Trewavas (Marillion), Portnoy (DT), and Neil Morse (Spock's Beard). If you like Transatlantic you will probably want to explore these other bands as well.
If you follow all these bands individually based upon enjoyment of the "supergroups", you will be busy for a very long time! (I would guess that the "home" bands of all the members of both OSI and Transatlantic have collectively released 75+ albums between them.)
Also, I would highly recommend you check out Echolyn. This is an amazing American Prog band with several releases. Unfortunately the CD releases are very difficult to find, but IIRC all their albums are available for download (I know, I know-crappy compressed sound quality, but don't deprive yourself of the great music simply on this basis). "Mei" and "Suffocating the Bloom" would be my top choices (but all their works are well worth hearing).
