The abbreviation for the genetic disease "Oversized Male Genitals", in which the genitals are both very long and very thick (commonly mistaken for "Oh my God!").
OMG sufferer: Dad I have something to tell you.
Father: OMG! You're gay!
OMG sufferer: No Dad... I have an OMG.
Father: I knew that you would get that from me.
Father: OMG! You're gay!
OMG sufferer: No Dad... I have an OMG.
Father: I knew that you would get that from me.
by IGotaOMG September 07, 2009