OB, a Latin preposition, signifies primarily, in front, before, and hence against, towards; as in objicio, to object, that is, to throw against. It has also the force of in or on; as in obtrude. In composition, the letter b is often changed into the first letter of the word to which it is prefixed; as in occasion, offer, oppose.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI