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.