No, the oxygen just simulates that the ship has made the run between the station attacked and the attacker's staging base. So, say a station requires 40,000 troops to take it, and you have 10 trains going back and forth, than those trains would all make 4 trips between the staging base (buying oxygen) and the invaded base (selling the oxygen). The referee would watch to make sure no ships just don't keep the oxygen, fly away a bit, and come back. Also, that referee would also count the trips each has taken and says when the base has been taken. So the trains should arrive and dock with oxygen on them, and leave with none.