Ive been using a tree farm, about 9 oaks quite tightly spaced with O2 collectors all round and running off a bank of 6 hydrogen generators from mekanism. Its enough to run my 12 O2 bubbles and plenty of power left over for other devices. Be warned that long large numbers of o2 pipes seems to cause a lot of stress on your server, specifically the game starts "pausing" for longer and longer periods the bigger the network gets. The only way I have solved this is by separating my O2 generators into small groups of trees (hidden in underground bunkers with stone torches for lighting) each feeding a separate O2 bubble.
For real space saving, make a stack of O2 collectors with power up one side and O2 pipe on the other, then surround with leaf blocks taken using shears. Check each generator as you work to ensure it is at 100% efficiency, adding extra layers if they come up short. These can easily be concealed under ground.
O2 bubbles seem to be working better than sealers as it is easy to oxygenate tall buildings where airlocks are hard to incorporate into the design. Bubbles also seem to generate far less of the stuttering effect than sealers.