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]

WordNet (r) 3.0 (2006):

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,
         Ob River]
    2: the branch of medicine dealing with childbirth and care of
       the mother [syn: obstetrics, OB, tocology, midwifery]