Optimus Prime is usually depicted as being a member of an ancient Transformers race called the Dynasty of Primes, often receiving the title "The Last Prime" in many stories, in which he is depicted as being the last of the Primes. In the Transformers: Covenant of Primus, it was established that Optimus Prime was the last born of the original Thirteen Transformers. It was his unique spark and his inspiring reassurance that "All are One" that allowed the Primes to rally and succeed in their battle against the Chaos Bringer Unicron. When tragedy at last ended the era of the Primes and brought forth the new race of lesser descendant Transformers he alone chose to be reborn in the Well of All Sparks as one of them, that he might know them and their needs more completely. All memory of his past life gone, he took the name "Orion Pax" and sought his way like any other robot on the new world becoming Optimus Prime once more when receiving the Matrix of Leadership when Cybertron faced a new enemy in his former friend, Megatron and his army of followers, the Decepticons. This brings a Great War to their planet of Cybertron. Optimus's origins and personality can vary depending on which "universe" he's seen in. This origin is the most consistent between the various incarnations. Further differences are listed in the respective sections below.
In one reality, while the Autobots were boarding the Ark, Optimus voiced his lack of confidence in his leadership, before welcoming aboard Geosensus, who was incapable of transforming to robot mode. Four million years later, Prime and Prowl found the Ark was under attack and defended it with deadly force. In examining the wreckage of the attackers, they realised that the robot Optimus had destroyed was not a Decepticon but Geosensus, who had forced himself to transform in an attempt to fight off the Decepticons. Though Prime was willing to take the rap, Prowl insisted on covering up the mistake to protect Optimus's image as a leader. Parts
In "A Fistful of Energon", Optimus Prime is informed by Ultra Magnus about Starscream's escape from prison. He is quite upset when he has heard that Prowl set out in search for him on his own. He orders Prowl to return, but he disobeyed him. Later, Prowl is bound and abandoned by Lockdown, who wants to get the bounty that Megatron had set on Starscream. But Optimus Prime, together with Bulkhead and Ratchet, succeed in finding and releasing Prowl.
When Sideswipe was accused and convicted of destroying the Nova Suspension Bridge, Optimus was infuriated that one of his own Autobots could commit such a heinous act. Optimus was tasked to hunt down and deactivate Sideswipe, though the task proved to be easy as Sideswipe did not run, hide or resist. After Optimus had deactivated Sideswipe, he was approached by Prowl, who pointed out that Sideswipe's actions made little sense, as he had committed the crime in broad daylight before many witnesses. Agreeing that the Decepticons might be behind the situation, Optimus allowed Prowl to take a team of Autobots to Carob Island to clear Sideswipe's name while Optimus and Jazz stayed behind to continue working on human/Autobot relations. Prowl returned with evidence that it was indeed a Decepticon-controlled clone of Sideswipe that had committed the crime. Optimus then presumably reactivated Sideswipe when his name was cleared. Deadly Paradise