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-.

Source: Webster's Revised Unabridged Dictionary (1913)