The Omega Sentinels were a mass-produced line of weapons of mass destruction sharing the same body-type as Omega Supreme. They were created as part of Project Omega. Prior to the Sentinels proper, at least four prototypes were created. The 1/12th scale Mark IV prototype, Omega S.P.R.E.E.M., is an example.
In "Transform and Roll Out", Optimus Prime viewed a history vid of the Great War that featured towering Transformers with cannons on their left arms and transparent faceplates. Though they strongly resemble the Sentinels, they differ in many details from their depiction in The AllSpark Almanac II. Either these are some other towering robots we don't know about, or the vid is not completely accurate.
As you can see, Blackout is very good at destroying Sentinels.
Most of the Omega Sentinels were planned to make a cameo in "TransWarped", but their scene was cut for time.