Give vents a small about of oxygen storage that can only be filled by depressurising a room. Overflow goes to tanks as normal.
Currently most airlocks players create are broken because if the oxygen tank is already full it never depressurizes - so the "depressurised" event in the vent never triggers. There are work arounds such as attaching a oxygen tank to a vent without adding it to the conveyor network, but this seems messy and if it is already empty you get the opposite problem - the pressurise event never fires. A better option would be changing the vent behavior so that the vent itself can store a small amount of oxygen itself, but this can only be filled by depressurizing a room. If this storage (say enough for a 3x3 room) is full the overflow goes to the next available oxygen tank.. Another possible solution is a tickbox on an oxygen tank that designates it is only allowed to be filled by airvents depressurising that way you can maintain full tanks of air, but dedicate a tank to serving airlocks.