Made popular by a talking cat, it is an expression made to show exasperation or surprise.

Alternatively, it can be used after you tell a joke. Like a "oh, snap!" type of thing.

A game named after the phrase spoken during play. An individual stands in a potentially dangerous location and counts how many times he can say "Oh Long Johnson" before moving out of harm's way. A primitive form, involving a train, was made famous in the 1986 film Stand by Me.