Best used when making fun of some poor unsuspecting fool. When he makes his best O smile (smiling and forming a big O with the lips), you tell everybody around you that he's good at giving a blowjob, and that there is your proof.
Guy # 1: Show me that O smile you homo!
Guy # 2: *O smile*
Guy # 3: Dude you totally sucked me off last night, look at that talent! Easy on the teeth!