Old Test. Old Testament.
O.M. Old Measurement.
omn. hor., omni hora=Every year.
onomat. Onomatopœia.
o/o. Percent.
O.P. Old Price; Ordinis Prædicatorum=Of the Order of Preachers (or Dominicans).
o.p. Out of Print.
Op. Opera.
op. Opposite; Opus=Work.
op. cit., opere citato=In the work cited.