OMC
O.M.C=Over mature child
Used to poke fun at someone who has a mature face but likes things normally associated with children or acts childishly.
Ben: Glenn, stop being so lame and childish, stop telling me about your adventures about you and your power ranger figurines, for god's sake, you are already 21.

Glenn: Mind your own business.

Ben:Whatever, how OMC can you get?