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1.
Referring to the One Star Fairy in the CBS forums. An O.S.F. is a Phaenomenon occurence in which every post is rated 1 star out of a possible 5. There is no actual member who is the O.S.F. it is just a speculated phantom. It is mainly associated in the NCIS forums.
I think the O.S.F. has been through here today.

or

The O.S.F. has landed.
by Random Sightings April 16, 2010
 
2.
OSF
abbreviation for Obersturmbannführer; A nazi party rank used by the SS and SA in the second world war
All hail the OSF
by Her Kommisar March 06, 2011
 
3.
OSF
Old scrotum face, coined by Chris Hovey a spurs fan to describe Harry Redknapp.
That OSF is useless!
by scarls June 28, 2012
 
4.
OSF
OHHHH SHIIIT FUUUCK
"did you fsu today?"
"osf i forgot"
"FSU!!"
"OSF!!"
by exus345 July 10, 2008
 
5.
OSF
my former place of employment, also stands for Only Stupid Fuckers
Only stupid fuckers worked at OSF
by cmac44 February 12, 2007
 
6.
OSF
One Suave Fuck
Look at that dude, he thinks he's OSF
by Doc23 August 27, 2010