Omega Supreme, huge, really really big, and did I mention frigging gigantic? His head transforms into a tank while the rest of him becomes tracks and a giant rocketship. He's one of the largest Autbots around and was built as a guardian for Crystal City. It was a peaceful time for Omega Supreme and this behemoth even had friends such as the Constructicons(see Gestalt page .)
Of course this couldn't last forever...
The Decpeticon leader known as Megatron used a device known as the robosmasher to corrupt the Constructicons and make them part of the decepticon team. Forced to become Decepticons, the Contstructicons were granted a special ability, they could all join together to form single larger decepticon known as Devastator.
Unfortunately for Omega Supereme...
He had know idea that Megatron had done this to his buddies. He learned the hard way of what happened to them when the Constructicons double crossed him.
The Constructicons came to Omega saying that another city was under attack, so he left his post to go help the city under seige. When he arrived he found that there was no problem, so he hurried back to his post only to find the city was destroyed by the Constructicons. Omega Supreme then knew that the Constructicons, who were once friends, were now enemies, or at least would be if he couldn't help them somehow.
Something had to be done...
So, he decided he was going to attempt to return the Constructicons to their original state. Unfortunately, he had no idea that Megatron's work was irreversible. Omega went about his business trying to retore them, and thought he had until the Constructicons combined together to form Devastator. In the mean time Megatron had his robosmasher try and reprogram Omega, so like the Constructicons, Omega himeself would be a Decepticon. It didn't work though, and in the process the robosmasher was destroyed.
Sadly...
The robosmasher, though unsuccessful at turning Omega Supreme into a Decepticon, did have other repercussions. What it wound up doing was leaving Omega with deep animosity towards the Constucticons and not feeling much else in the way of emotions other than hatred and anger. Because of this Omega became obsessed with finding the Constructicons, and wheen he learned that they had gone to Earth, he went decided it was time to go there himself.