The Collaborative International Dictionary of English v.0.48:

Modus operandi \Mo"dus op`er*an"di\[L.],
   manner of operating. Often abbreviated to MO
   [1913 Webster]

WordNet (r) 3.0 (2006):

modus operandi
    n 1: an unvarying or habitual method or procedure [syn:
         routine, modus operandi]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "modus operandi":
   MO, algorithm, approach, attack, course, fashion, form, guise,
   line, line of action, lines, manner, manner of working, means,
   method, methodology, mode, mode of operation, mode of procedure,
   order, practice, procedure, proceeding, process, routine, style,
   system, tack, technique, the drill, the how, the way of, tone, way,
   wise