Ok, summary time. The word or acronym M.O.S. can have several meanings:

1. Member of the Opposite Sex
2. Mutual Oral Sex (see 69)
3. Make out session
4. "Mom over shoulder", a warning in chatrooms or IMs
5. Ministry of Sound, a nightclub in London UK
6. A short form for mos def, which is a short form for "most definitely".
7. Military Occupational Specialty (US Army)

M.O.S. can also be an abbreviation of a shitload of other terms that are mos def not used in slang. Including Metal-Oxide Semiconductor, Made of Steel, Marine Observation Satellite, Medical Outcomes Study, Member of Service, Mode Of Shipment etc.
Err... recently 2 London grrlz on aim:

A: "So tell me what happened after the party at MoS last night!"
B: "Ok, I was just about to leave when I met this really handsome MOS.
He told me his MOS is Infantryman in the US Army. Guys in uniform just turn me on!"
A: "Sweet, and then? Did you make out with him?"
B: "Yes! We had a good mos in the club and then he asked me if I wanted to come to his place, I said 'mos!'"
A: "Really, so did you...?"
B: "Yeah, when we got there, we started to have some real good MOS..."
A: "Wait, MOS!"
B: "Ok, ttyl ;)"
by 007 Bond November 20, 2006
Get the mos mug.
stands for modus operandi which translates as method of operation
Can't you see why i'm so mean? if y'all leave me alone, this wouldn't be my mo
by Donovan June 12, 2003
Get the mo mug.
a cross between an idiot and a moron
originally from the Boston, Mass. area.
I really made a mo-mo of myself.
by Mel-C March 11, 2005
Get the mo-mo mug.
slang that means a lot or in an excessive amount.
Jeff got mo' booze for the party last night.
by Joe Ram July 20, 2006
Get the mo' mug.
Short for homo. Mainly used by the gay community when referring to each other. Rarely used as a homophobic insult.
Is he really a 'mo?
No way!
by sack jaffery July 7, 2009
Get the 'mo mug.
Think of the dumbest person you know. Thats the best way I can describe the term Mo-Mo
Did you see how drunk Joe got last night? What a Mo-Mo. For short, "Hey Mo, while ya up, get me a beer."
by john mc May 21, 2005
Get the Mo-Mo mug.
Military Occupational Specialty (MOS).
My MOS is Infantryman (11B).
by discharge March 3, 2005
Get the MOS mug.