This image by clu-art shows Optimus Prime as a Green Lantern, I’m going to admit my knowledge of the Green Lantern mythos is not what it could be, but I think I’m right in thinking that the Green Lantern Corps. have members from hundreds of planets protecting the galaxy from whatever it is the Green Lantern Corps. fight (Probably Red Lanterns or something like that, didn’t guy Gardener become a yellow lantern, or something like that?).
There is no reason why there couldn’t be a Cybertronian Green Lantern and who better than Optimus Prime?