Source : WordNet®

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}]

Source : Free On-Line Dictionary of Computing

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}]