Ob-

CIDE DICTIONARY

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]

OXFORD DICTIONARY

Ob-, prefix (also oc- before c, of- before f, op- before p) occurring mainly in words of Latin origin, meaning:
1 exposure, openness (object; obverse).
2 meeting or facing (occasion; obvious).
3 direction (oblong; offer).
4 opposition, hostility, or resistance (obstreperous; opponent; obstinate).
5 hindrance, blocking, or concealment (obese; obstacle; occult).
6 finality or completeness (obsolete; occupy).
7 (in modern technical words) inversely; in a direction or manner contrary to the usual (obconical; obovate).

Etymology

L f. ob towards, against, in the way of