Substitutional hydrogen at oxygen site (H-O) is well-known to be a robust source of n-type conductivity in ZnO, but a puzzling aspect is that the doping limit by hydrogen is only about 10(18) cm(-3), even if solubility limit is much higher. Another puzzling aspect of ZnO is persistent photoconductivity, which prevents the wide applications of the ZnO-based thin film transistor. Up to now, there is no satisfactory theory about two puzzles. We report the bistability of H-O in ZnO through first-principles electronic structure calculations. We find that as Fermi level is close to conduction bands, the H-O can undergo a large lattice relaxation, through which a deep level can be induced, capturing electrons and the deep state can be transformed into shallow donor state by a photon absorption. We suggest that the bistability can give explanations to two puzzling aspects.