Omnicon - A sub-group of Autobots whose purpose is to mine and fashion Energon into usable forms.
Optic Sensor - A Transformer term for eyes.
Optimus Prime - Leader of the Autobots. Has died and returned to live many times in various continuities. In Japan, he was known as "Convoy".
Oracle - A super computer in the lower levels of Cybertron. This computer has a direct connection with Vector Sigma, and has the ability to reformat Transformers into new forms.
Optronix - The name of the robot destined to become Optimus Prime starting with the Dreamwave "The War Within" continuity.
Orion Pax - Young, idealistic robot damaged by Megatron and rebuilt into Optimus Prime by Alpha Trion.