Definition of ob- in US English:

ob-

prefix

  • 1Denoting exposure or openness.

    ‘obverse’
    1. 1.1 Expressing meeting or facing.
      ‘observe’
  • 2Denoting opposition, hostility, or resistance.

    ‘obstacle’
    1. 2.1 Denoting hindrance, blocking, or concealment.
      ‘obviate’
      ‘obliterate’
  • 3Denoting extensiveness, finality, or completeness.

    ‘obsolete’
    ‘obdurate’
  • 4(in modern technical words) inversely; in a direction or manner contrary to the usual.

    ‘obconical’

Usage

Ob- occurs mainly in words of Latin origin. It is also found assimilated in the following forms: oc- before c (occasion); of- before f (offend); op- before p (oppress)

Origin

From Latin ob ‘towards, against, in the way of’.