Think Monday Club Bar at UpStairs serves late. Ditto Noir. Bar at Rialto used to, but don't know if it still does. Personally, I'd prefer all of the above to the lounge at OM, which is just too precious for schlubs like myself.
Actually, I think there are really quite a few late-night options in the Square, depending on the type of scene you're looking for. It's not one of the 'hoods that goes black at 10.