O

Obliterators,

Two transformers from the second generation. One Autobot; Spark, and one Decepticon; Colossus.

Octane,

Decepticon Triple Changer. Transforms into a plane or a truck.

Octopunch,

Decepticon. Transforms into a crab. He is one of the Pretender Figures.

Off Road Patrol,

Autobot Micromasters. This is one of the Micromaster Patrols. The group is formed by:
Highjump
Mudslinger
Powertrain
Tote

Oiler,

Autobot Micromaster. Transforms together with Slide into a tanker. He is a member of the Metro Squad.

Omega Spreem,

Autobot Actionmaster. Transforms into a battledroid. He is one of the Actionmaster Elite Figures.

Omega Supreme,

Autobot Defense Base. Transforms into a rocket and it's launch pad. In the series before the series he was one of the strongest (THE strongest in my opinion) Transformers. He was one of the Guardian Robots on Cybertron. After the Constructicons destroyed Crystal City, Omega Supreme followed them to Earth.

Omnibots,

Group of three Autobots. They have only been available by mailorder. (Powerdasher was also one of the Transformers who were only available by mailorder, but he is not a member of the Omnibots.) The names of the Omnibots are:
Camshaft
Downshift
Overdrive

Onslaught,

Decepticon. Transforms into a military vehicle. He is the leader of the Combaticons.

Onslaught,

Decepticon from the second generation. Second release of Onslaught.

Onyx Primal,

Predacon. Comes from the Beast Wars. Transforms into an animal. He was released at BotCon '96.

Optimal Optimus,

Maximal. Comes from the Beast Wars. He is a Transmetal Figure, transforms into a hypersonic jet or an all-terrain ground assault carrier.

Optimus Primal,

Maximal. Comes from the Beast Wars. Maximal leader, transforms into a gorilla.

Optimal Primal,

Maximal. Comes from the Beast Wars. He is a Transmetal Figure, transforms into a gorilla, his feet can turn into a hoverboard.

Optimus Primal & Megatron,

Maximal and Predator. Comes from the Beast Wars. Optimus Primal transforms into a bat, Megatron into a crocodile.

Optimus Prime,

Autobot Actionmaster. His truck transforms into a jet, his trailer into a battle station.

Optimus Prime,

Autobot. Optimus Prime himself transforms into a truck, his defense base transforms into the trailer. Optimus Prime and the trailer can combine to form a much larger robot. Hi-Q transforms into his engine. He is one of the Powermasters.

Optimus Prime,

Autobot. He is the leader of the Autobots. In search for new energon supplies he took many Autobots with him on a journey into deep space. Just after they left Cybertron their shuttle was attacked by Decepticons (See More Then Meets The Eye, part 1). Both Autobots and Decepticons were on board the Ark when it crashed on Earth. Many milions of years later both groups were reactivated and Optimus Prime led the Autobots in their struggle against the Decepticons. After almost twenty years on Earth, he and Megatron battle each other to the death (See The Movie). Optimus Prime passes the Matrix over to Ultra Magnus before he dies. After some time Rodimus Prime becomes leader of the Autobots. One time the Quintessons try to use the body of Optimus Prime to trap Rodimus Prime and the Autobots, but they fail. Later when a plague threatens the universe, Rodimus Prime orders Sky Lynx to find a Quintesson to restore Optimus Prime. The Quintesson succeeds and Optimus Prime leads the Autobots once again, he even stays leader till the end of the series.
Optimus Prime was originally a robot named Orion Pax, but at the start of the Cybertronian Wars he was injured by Megatron. Alpha Trion rebuilt Orion Pax into Optimus Prime. Optimus Prime has also a girlfriend: Elita One, she is the leader of the Female Autobots.

Optimus Prime,

Autobot from the second generation. I have just heard that he exists, I don't know in what he transforms (probably a car, just like all the gobots that I know). He is one of the Gobots.

Optimus Prime,

Autobot from the second generation. On message boards he is sometimes referred to as "Laser Optimus Prime". The truck transforms into Optimus Prime, the trailer into a battlestation. Optimus Prime has a laser sword that really lights up (uses a battery) and the trailer has an airpowered rocketlauncher.

Optimus Prime,

Autobot from the second generation. In fact it is a second release of the original Optimus Prime. The trailer is now black instead of grey. There is an addition in the form of some sort of box that one can place on the front end of the trailer. This box can make sounds and lights (uses battery).

Optimus Prime,

Hero Optimus Prime. I have no data on him, he only appeared in the US. Just heard of his existence; it would be the same Transformer as the European generation 2 version of Sureshot (See pic there).

Optimus Prime,

Machine Wars Optimus Prime. I have no data on him. Just heard of it's existence

Optimus Prime,

Machine wars. Transforms into a trailer. He has the same shape as Eagle (=Thunderclash).

Orbulus,

Robot, inhabitant of the planet Lithone. When Unicron attacks the planet, at the beginning of the The Movie he is digested by Unicron. Only his friend Kranix is able to flee.

Orion Pax,

Autobot. Was one of the first casualties of the civil war on Cybertron. He was rebuilded into Optimus Prime by Alpha Trion (See War Dawn).

Outback,

Autobot. Transforms into a car. He is one of the Mini-Vehicles.

Over-Run,

Autobot Actionmaster. His helicopter transforms into a little battlestation.

Overbite,

Decepticon. Transforms into a monster. He is one of the Seacons.

Overdrive,

Autobot. Transforms into a car. He is one of the Omnibots.

Overflow,

Autobot Micromaster. Transforms together with Full-Barrel into a truck. Together they control the Autobot Headquarters.

Overkill,

Decepticon. Transforms from cassette into a monster. He is one of the Spy-Cassettes.

Overload,

Autobot Micromaster. Transforms into a truck. His trailer transforms into a jet. He is one of the Micromaster Transports.

Overlord,

Decepticon Overlord. Transforms into a stealth bomber and a tank. Two small robots accompany him. In the third season of the Japanese series he is one of the leaders of the Decepticons.

Override,

Autobot. Transforms into a motorcycle. He is one of the Triggerbots.