[quote name='proby']If I'm not mistaken, OPS+ requires the orginal OPS to play.[/QUOTE]
No, it's a stand alone game.
I picked mine up for $10 with the two PSP games $19.99 and under glitch at CC. Sadly, the glitch is dead.
Basically, its a multiplayer only game with a quick-play-style single player tacked on for fun.
Stolen from KiteDXX @ GameFAQs because I was too lazy to type all this shit out:
What is different between this game and the original MPO? - [QQ2]
-Story mode was removed.
-New "Infinity Mission" mode replaces single player.
-New soldiers, levels, weapons, and items.
-New online server.
-New online functions, such as manual text entry chat.
-New online maps.
-EXP system replaces dispatch system to gain soldier stats.
-New S2 and S3 weapon mastery stats.
-New Training mode.
-Some other minor things.
What is "Infinity Mission" and what do you do in this mode? - [QQ3]
Infinity Mission replaced single player's story mode. It's a short game mode where you simply move from point A to point B in several maps to reach the last stage. You can still recruit soldiers like before, but there are a lot of differences. The exits are randomized. You start with no weapons. There are a few twists in the form of "special missions" where you must complete a certain objective to progress, such as defeating all soldiers, avoiding alert mode, and surviving a certain amount of time. The last stage will always consist of defeating a certain number of soldiers. This mode is also how you gain EXP for your soldiers.
Also, you can do some neat stuff if you import your MGS: PO savefiles.
The game is so cheap, it will be hard for you to find a "deal" on this title.