The Collaborative International Dictionary of English v.0.48:

Operant \Op"er*ant\, a. [L. operans, p. pr. of operari. See
   Operate.]
   Operative. [R.] --Shak. -- n. An operative person or thing.
   [R.] --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

operant
    adj 1: having influence or producing an effect; "many emotional
           determinants at work"; "an operant conscience"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "operant":
   actor, agent, architect, author, conductor, creator, doer, driver,
   engineer, executant, executor, executrix, fabricator, functionary,
   handler, maker, manipulator, medium, mover, operative, operator,
   performer, perpetrator, pilot, practitioner, prime mover, producer,
   runner, steersman, subject, worker