The Collaborative International Dictionary
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-.
Douglas Harper's Etymology Dictionary
prefix meaning "toward, against, across, down," also used as an intensive, from Latin ob "toward, to, over against, in the way of, by reason of, about, before, in front of," from PIE root *epi, also *opi "near, against" (see epi-).
Etymology 1 pre. 1 (label en non-productive) Against; facing; a combining prefix found in verbs of Latin origin. 2 (label en botany) Of a reversed shape. Etymology 2
pre. (label en Internet informal) obligatory; prepended to the name of a topic being mentioned to avoid accusations of being off-topic.