necro!
yes, a giant dome can be sealed with approximately 28 Oxygen Sealers.
Some tips:
* make sure you're using the correct latest Galacticraft - not the "stable" one, the latest has better Oxygen Sealers
* before you start, test a small room or dome with the same types of blocks, to make sure all blocks are sealed (some Carpenters Blocks for example can be hard to seal)
* make sure you have enough oxygen and electricity for all these sealers
* it's best to put your oxygen and electricity systems in the same chunk as the Oxygen Sealers or very close. Personally I would build power systems underground, directly below the Oxygen Sealers. If you put the oxygen or electric generators a long way away, like 100 blocks away or something, then they might not be loaded by the game when your dome is loaded, so there would be nothing to give the Oxygen Sealers the supplies they need.