No co-op in MOHAA, unfortunately. OFP does feature it and is very good at it. Other military titles with multiplayer co-op that I have tried online include Ghost Recon, Deadly Dozen 2, and Hidden & Dangerous, plus the Novalogic Delta Force series.
Non-military co-op shooters include SWAT 3 and the Rainbow 6 series.
Hope this helps! Online co-op is a lot of fun.