"In Ambush, two teams of mercenaries fight to deplete the available clone reserves of the other team within the allotted time"
In Ambush, teams are split into 2 groups of 16 mercenaries; with 50 clones on reserve. Ambush games have a maximum mission time of 15 minutes. Vehicles are not allowed in ambush, and installations do not spawn.
The only objective in Ambush is to kill everyone on the enemy team until their clone reserves are depleted. In the event that the mission timer expires, the team with the most clones left in reserve wins, and the other retreats.
Map sizes on Ambush games are typically very small (although some maps afford more space not covered by the Red Line) in order to concentrate the action.
Ambush OMS is slightly different from regular ambush in that instillations will spawn every 2 minutes, and vehicles are allowed to be used.
You cannot chose which type to play, as the queues of the two game modes are combined.