Merriam-Webster's Collegiate Dictionary:
New Latin, from Latin, in the way, against, toward, from ob
in the way of, on account of; akin to Old Church Slavic o, ob
inversely Example: obovate
Free On-line Dictionary of Computing:
/ob/ prefix Obligatory. A piece of netiquette
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.