The Omega Key was a device, used to gain acces to the core of Cybertron.
War for Cybertron
After Starscream reactivated the Energon Bridge, Megatron and Soundwave discussed a attack on Iacon to get the Omega Key. While Starscream and most of the other Decepticons fought in the main battle, a small group around Megatron, including Soundwave, Breakdown and Brawl (who was leading the small group, except for Megatron, Soundwave and Breakdown), infiltrated the City, battled Zeta Prime and got the key. After it started to blink, Zeta revealed, it wasn't the real key, but Megatron wouldn't have to worry, because the real key would come to him. Megatron and a few of his soldiers searched for it, until Omega Supreme destroyed their ships. Megatron, Soundwave and Breakdown battled their way to some Ion Cannons and shot Omega Supreme to the ground, where he was defeated. Megatron then forced him to open the Omega Gate and corrupted the Core of Cybertron with Dark Energon.
- As said above, the real key is Omega Supreme.