WordNet (r) 3.0 (2006):

go ahead
    v 1: proceed (with a plan of action); "He went ahead with the
         project" [syn: go ahead, plow ahead]

Moby Thesaurus II by Grady Ward, 1.0:

217 Moby Thesaurus words for "go ahead":
   John Hancock, OK, acceptance, admission, advance, advancement,
   advancing, adventuresome, adventuresomeness, adventurous,
   adventurousness, affirmance, affirmation, aggression, aggressive,
   aggressiveness, allowance, ambitious, ambitiousness, ameliorate,
   amend, antecede, approbation, approval, authentication,
   authorization, begin, blast away, blast off, career, certification,
   charter, come along, come before, come first, come on, commence,
   confirmation, consent, countersignature, course, cover ground,
   develop, dispensation, dive in, drive, drive on, driving, dynamic,
   dynamism, endorsement, enterprise, enterprising, enterprisingness,
   fall to, force, forceful, forcefulness, forge ahead, forward,
   forward motion, forwarding, front, furtherance, furthering, gain,
   gain ground, gather head, gather way, get ahead, get along, get to,
   get-up-and-get, get-up-and-go, getup, go, go ahead of, go all out,
   go along, go before, go fast, go forward, go in advance, go on,
   go-getting, go-to-itiveness, graduate, green light, grow better,
   gumption, have priority, head, head into, head the table, head up,
   headway, hustle, hustling, imprimatur, improve, initiative,
   jump off, kick off, lead, lead off, leave, liberty, license,
   look up, make good time, make head against, make headway,
   make progress, make progress against, make strides,
   make things hum, make up leeway, march, meliorate, mend, move,
   move forward, moving, nod, notarization, okay, oncoming, ongoing,
   onward, onward course, outrank, pass along, pass on, passage,
   patent, perk up, permission, permission to enter, pick up,
   pitch in, plunge into, precede, press on, proceed, proceeding,
   progress, progressing, progression, progressive, progressiveness,
   promotion, push, pushful, pushfulness, pushiness, pushing,
   pushingness, pushy, rank, rate, ratification, release, roll,
   rolling, rolling on, rubber stamp, sanction, scramble, seal,
   send off, set about, set in, set out, set sail, set to, shape up,
   shoot ahead, show improvement, sigil, signature, signet, skyrocket,
   special permission, spirit, spunk, stamp, stamp of approval,
   stand first, start, start in, start off, start out, stem,
   step forward, step lively, subscription, take off, take precedence,
   the nod, ticket, ticket of admission, travel, turn to,
   up-and-coming, up-and-comingness, usher in, validation,
   venturesome, venturesomeness, venturous, venturousness, visa, vise,
   vouchsafement, waiver, warrant, way