O&S is the abbreviation of the duo bosses in
the game "Dark Souls", Ornstein and Smough, whom guard the Chamber of the Princess in Anor Londo.
This abbreviation is used by
lazy people, whom
don't bother writing the full names when looking up guides on how to beat them.
The O&S bossfight is sometimes referred to as the hardest and douchiest boss battle in the game.
Nick: Stop screaming Dan! What are you doing?!
Dan: O&S are fucking annoying as
shit. Ornstein
fucking stabs me, then Smough rapes me with his
hammer!