The Collaborative International Dictionary of English v.0.48:

Open-mouthed \O"pen-mouthed`\, a.
   Having the mouth open; gaping; hence, greedy; clamorous.
   --L'Estrange.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "openmouthed":
   advertent, agape, aghast, agog, ajar, alert, all agog, all ears,
   all eyes, amazed, assiduous, astonished, astounded, at gaze,
   attentive, aware, awed, awestruck, bawling, beguiled, bewildered,
   bewitched, blatant, boanergean, boisterous, brawling, breathless,
   burning with curiosity, captivated, careful, clamorous,
   clamoursome, concentrated, confounded, conscious,
   consumed with curiosity, crying, curious, dehiscent, diligent,
   dumbfounded, dumbstruck, earnest, enchanted, enraptured,
   enravished, enthralled, entranced, fascinated, finical, finicking,
   finicky, flabbergasted, gaping, gauping, gazing, ghoulish, gossipy,
   heedful, hypnotized, in awe, in awe of, inquiring, inquisitive,
   intense, intent, intentive, interested, itchy, lost in wonder,
   loudmouthed, marveling, mesmerized, meticulous, mindful, morbid,
   morbidly curious, multivocal, nice, niggling, noisy, observant,
   observing, obstreperous, on the ball, on the job, open-eared,
   open-eyed, oscitant, overcurious, overwhelmed, popeyed, prurient,
   puzzled, quizzical, rapt in wonder, regardful, ringent,
   scopophiliac, screaming, shouting, slack-jawed, spellbound,
   staggered, staring, strident, stupefied, supercurious, surprised,
   thunderstruck, under a charm, vociferant, vociferating, vociferous,
   voyeuristic, watchful, wide-eyed, wonder-struck, wondering,
   yammering, yapping, yawning, yelling, yelping