Hovering silently, O never says a single word of dialogue throughout the series. His ship appears similar to a jellyfish, and both share the ability to absorb energy and send out devastating pulses in response. O can also take a larger, more combat-ready form with actual limbs.
Long ago, when Satis faced Canaletto in combat, he called to O for help. However, O did not respond to the request. All the way to the present, O felt incredibly guilty that he did not go back to help Satis. So, in an act of repentance, he defended Molly, Jordan and Prince Aikka when they were attacked by the two statues guarding the Temple of the Heart. Ultimately, he could not defeat the two monstrosities -- However, using his energy-absorption abilities, he was able to generate a massive explosion that took them down with him.
O is a very powerful competitor, fast and nimble. He has the power to absorb energy and release it in the form of very destructive pulses. Because Satis called O to help him in battle against Canaletto, we can deduce that he is both very old and extremely powerful.