Source : Webster's Revised Unabridged Dictionary (1913)

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, ovate. Ob- is commonly
   assimilated before c, f, g, and p, to oc-, of-, og-, and op-.