/o\ is an emoticon for someone protecting his head with his arms. Often used in French communities where it is synonymous to "PATAPAI" ("don't hit me !").
Jack> Hello Europe, not too angry to have to wait one year more than the US for the PS3 ?
Jack> /o\
A smiley, the opposite of \o/.
The person is still putting their arms in the air, but their shoulders seem to be dislocated in this version.
Totally pointless and doesn't relate to anything.
Random Person 1: I won $63.85 in the lottery!! \o/
Random Person 2: /o\ - My ****ing arms are broken?!
A smiley of a cheering guy throwing his hands in the air, expressing great happiness. In a rare case, it can portray someone throwing his hands in the air in exasperation or panic.