Oa is the home planet of the Guardians, little blue folks with big heads and red robes who float around screwing up the universe and then send out Green Lanterns to fix it or make it worse or something. Here is what I came up with in Photoshop for their central location.
One major problem from an art direction perspective, with GLAS and anything Green Lantern in general, is the overabundance of green. It can be difficult to come up with nice color palettes when everybody and everything in the scene is glowing green. Matters were not made better with the introduction of Red Lanterns, as we now had the world's weirdest Christmas to deal with.
Anyway, I tried a few things with Oa. Some were just different times of day, others were different takes on the palette. I think we went with the greenest one.