WordNet sense 1 (used of mouth or eyes):

open, opened

WordNet sense 2 (an involuntary intake of breath through a wide open mouth; usually triggered by fatigue or boredom):

yawn, oscitancy, oscitance

WordNet sense 3 (showing lack of attention or boredom):

drowsy, oscitant

WordNet sense 4 (showing a lack of attention or care):

inattentive