[1] : The Collaborative International Dictionary of English v.0.48

ob- \ob-\ l. ob, prep. cf. epi-.
   a prefix signifying to, toward, before, against, reversely,
   etc.; also, as a simple intensive; as in oblige, to bind to;
   obstacle, something standing before; object, lit., to throw
   against; obovate, reversely, or oppositely, ovate. ob- is
   commonly assimilated before c, f, g, and p, to oc-, of-, og-,
   and op-.
   1913 webster
see also:
epi- 

[2] : WordNet (r) 2.0

ob
     n 1: a major river of western siberia; flows generally northward
          and westward to the gulf of ob and the kara sea syn: ob
          river
     2: the branch of medicine dealing with childbirth and care of
        the mother syn: obstetrics, tocology, midwifery
see also:
ob river obstetrics tocology midwifery 

[3] : Jargon File (4.3.1, 29 Jun 2001)

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.


see also:
netiquette winnitude 

[4] : The Free On-line Dictionary of Computing (27 SEP 03)

ob-
     
        /ob/ prefix 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.
     
        jargon file
     
     
see also:
netiquette winnitude jargon file 

Ob- >> O Words
Ob-, definition of term: Ob-
ob-_pag1.html