is shorthand for the now commonly used phrase, "Oh My God, Lady Gaga!," which is used when referring to the pop sensation herself, her songs, or anything that pertains to Lady Gaga.
You and your friends are partying at a nightclub. All of a sudden, the DJ starts playing the Chu Fu Remix of Bad Romance (or any other song by Lady Gaga). You and all of your friends start screaming "OMGaga! OMGaga!" to show your love and support for Lady Gaga.