ob- Also o-, oc-, of-, op-, and os-.
Towards, to, on, over, or against.
[Latin ob, towards, against, in the way of.]
Most words in this prefix are based on Latin words which already contain it, often in a figurative sense. Examples include obtrude (Latin trudere, to push), to become noticeable in an unwelcome or intrusive way; obverse (Latin vertere, to turn), the side of a coin or medal bearing the head or principal design; obsolete (Latin solere, be accustomed); obese (Latin esus, the past participle of edere, to eat); obey (Latin audire, hear); obstacle (Latin stare, stand).
The prefix also appears in assimilated forms in which it is reduced to o- followed by a duplication of the first letter of the stem. It becomes oc- before stems beginning in c (occasion, occlude), of- before f (offend), and op- before p (oppress, opponent). Without the duplicated consonant, it sometimes becomes os- (ostensible) and sometimes just o- (omit).
The prefix appears in a few specialist technical terms. These may be linked to obverse, in which it seems to mean ‘inversely’; the idea is of something that is in a direction or manner that is contrary to the usual. Examples are obduction, a term in geology for the movement of the edge of a crustal plate over the margin of an adjacent plate, the opposite of subduction; obcordate, used especially of a heart-shaped leaf that has its attachment at the apex; and obovate, of something that is ovate, like the outline of an egg, but with the narrower end at the base.