To sum up what Heners said:
You have three possible ways to get the right Opflash:
1) Original Operation Flashpoint AND Resistance Expansion Pack
2) Operation Flashpoint Gold Edition AND Resistance Expansion Pack
3) Operation Flashpoint Game of the Year Edition
There is something else called Red Hammer. ?This is included in the Gold Edition and the Game of the Year Edition, but is not necessary for multiplayer games. ?It is just an additional single player campaign. ?It says it adds new units, but the same units are also included in Resistance, which is necessary.