O&S 

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!

O&S by xX_PussyDestroyer_ShagsUMum_Xx April 9, 2016

O's 

ounces,particularly of crack

i cooked up ten o's last night.

O's by Devon W. February 15, 2006

o.s. 

abbreviation of "one's self" sometimes used in dictionaries

French dictionary:
s'occuper
- to occupy o.s.
- to keep o.s. busy

o.s. by TomParys March 6, 2013

O.S 

Oral Sex

That bitch gave me O.S Last night

O.S by AC November 11, 2003

O.S. 

A.K.A Original Sin

A clan in the popular mobile game Vampires Dark Rising or VDR for short. Founded by the Leader Antimus "Ant", they are an awesome group who treat each other like family and have beat Venom one of the largest clans after a year long war.

O.S. is an amazing clan

O.S. by DF2 September 22, 2017

o's 

common baltimorian nickname for the Orioles, the best team in baseball history, arch-enemies of the New York Yankees, also emphasized heavily in the national anthem

O's baby... O-R-I-O-L-E-S

o's by J May 5, 2005

o.s. 

short for opposite sex

In my free time, I do fun things with multiple members of the o.s.

o.s. by KatyxToxic January 27, 2008