WordNet (r) 3.0 (2006):

OK
    adv 1: an expression of agreement normally occurring at the
           beginning of a sentence [syn: very well, fine,
           alright, all right, OK]
    adj 1: being satisfactory or in satisfactory condition; "an all-
           right movie"; "the passengers were shaken up but are all
           right"; "is everything all right?"; "everything's fine";
           "things are okay"; "dinner and the movies had been fine";
           "another minute I'd have been fine" [syn: all right,
           fine, o.k., ok, okay, hunky-dory]
    n 1: a state in south central United States [syn: Oklahoma,
         Sooner State, OK]
    2: an endorsement; "they gave us the O.K. to go ahead" [syn:
       O.K., OK, okay, okey, okeh]

Moby Thesaurus II by Grady Ward, 1.0:

290 Moby Thesaurus words for "OK":
   John Hancock, Roger, absolute, absolutely, accede to, accept,
   acceptable, acceptance, accord, accord to, accredit, accurate,
   ace-high, acquiescence, adequate, adherence, admiration, admire,
   admissible, admission, admit, affirm, affirmance, affirmation,
   affirmative, affirmative voice, agree to, agreeable, agreed,
   agreement, all right, allow, allowance, alright, alrighty, amen,
   approbation, approval, approve, approve of, as you say, assent,
   assuredly, authenticate, authentication, authorization, authorize,
   autograph, aye, bad, bang-up, be willing, benediction,
   better than nothing, bless, blessing, bonzer, boss, bully,
   but good, by all means, certainly, certification, certify, charter,
   compliance, condescend, confirm, confirmation, connivance,
   connive at, consent, consent to silently, cool, corking, correct,
   cosign, countenance, countersign, countersignature, crackerjack,
   da, dandy, dead right, decent, deign, delicious, dispensation,
   dispense, ducky, eagerness, endorse, endorsement, esteem, exactly,
   fab, fair, fairish, faultless, favor, favorable vote, fine,
   fine and dandy, flawless, gear, give consent, give leave,
   give permission, give the go-ahead, give the imprimatur,
   give the word, give thumbs up, go along with, go-ahead, good,
   good enough, goodish, grant, great, green light, groovy,
   have no objection, hear, heavy, hold with, hot, hunky-dory,
   imprimatur, indeed, indeedy, initial, ja, jam-up, just, just dandy,
   just right, just so, keen, keep in countenance, leave, let,
   letter-perfect, liberty, license, mais oui, make possible, marvy,
   mean, meticulous, moderate, most assuredly, naturally,
   naturellement, neat, nifty, nobby, nod, nod assent, not amiss,
   not bad, not half bad, not refuse, not so bad, notarization,
   notarize, of course, okay, oui, out of sight, pass, pass on,
   pass upon, passable, patent, peachy, peachy-keen, perfect,
   permission, permission to enter, permit, positively, precisely,
   presentable, pretty good, promptitude, promptness, proper, quite,
   rather, ratification, ratify, readiness, really, release, respect,
   respectable, right, righto, ripping, rubber stamp, rum, sanction,
   satisfactory, say amen to, say aye, say the word, say yes,
   scrumptious, seal, seal of approval, second, sigil, sign,
   sign and seal, signature, signet, slap-up, smashing, solid,
   something else, special permission, spiffing, spiffy, stamp,
   stamp of approval, straight, straight-up-and-down, stunning,
   submission, subscribe to, subscription, sufficient, support, sure,
   sure thing, surely, swear and affirm, swear to, swell,
   take kindly to, tenable, the nod, think well of, ticket,
   ticket of admission, tidy, to be sure, tolerable, tough, truly,
   undersign, underwrite, unexceptionable, ungrudgingness,
   unloathness, unobjectionable, unreluctance, uphold, validate,
   validation, very well, viable, view with favor, visa, vise, voice,
   vote, vote affirmatively, vote aye, vouchsafe, vouchsafement,
   waiver, warrant, well and good, why yes, willingness, wink at,
   wizard, workmanlike, yea, yea vote, yeah, yep, yes, yes indeed,
   yes indeedy, yes sir, yes sirree, yet, yield assent