It is often difficult to recognize ob- as a prefix because of its variants and its lack of consistent meaning.
The ‘towards’ or ‘against’ meaning can be seen in words such as obtrude, literally, to thrust towards, and so to force an opinion ‘upon others’. Object contains the idea of throwing against. From this comes the meaning ‘to criticise, oppose’, as when you object to someone’s behavior or to a statement. An object, a thing, was originally something thrown in the way and thus something tangible, or a goal or purpose.
An easier etymology is that for opportunity. An opportune wind was one that blew you towards the port, and thus a favourable one.
Ob- can also act as an intensifier. This comes out in obnoxious, ‘thoroughly poisonous’; oppress, ‘to press or lie heavily upon’; obtain, ‘to take (firm) hold of’; and obdurate, ‘thoroughly hardened’, and so ‘hardened against persuasion, unyielding; hard-hearted or pig-heades’.
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