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]

The Jargon File (version 4.4.7, 29 Dec 2003):

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.


The Free On-line Dictionary of Computing (30 December 2018):

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]