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From 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]