Well, OFP2 is nothing like Halo ODST. Operations Flashpoint is, basically, a shooter that tries to be very, very realistic. Don't expect fast paced actions, because it's more about real warfare. That means walking and waiting and spying befire taking the actions you would immediately thown into in games such Halo or Modern Warfare.
You can die if someone shoots you in the head... or an artery, because you can bleed to death to. You have to command a squad, and ordering a frontal attack is just sending men to a certain death. It requieres some strategy (haven't played it yet, so I can't tell how much). The point is, you have to be very, very careful.
It's far more open, too. The game is a huge sandbox in which you are sent in among missions. It's not like you can roam freely, but you just have so much space to maneuver.
So, that's basically it. Hope it helps.