Ob-

Ob- /ob/ pref.  Obligatory.  A piece of {netiquette}
   acknowledging that the author has been straying from the
   newsgroup's charter topic.  For example, if a posting in alt.sex is
   a response to a part of someone else's posting that has nothing
   particularly to do with sex, the author may append `ObSex' (or
   `Obsex') and toss off a question or vignette about some unusual
   erotic act.  It is considered a sign of great {winnitude} when
   one's Obs are more interesting than other people's whole postings.