The acidity of an organic compound is affected by the relative stability of its conjugate base or its ion. In this case, the conjugate base of phenol is the phenoxide ion which is the phenol with the absence of the H atom on the OH. The lone pair on oxygen is able to delocalise into the benzene ring which forms a p-p orbital overlap otherwise known as resonance. This disperses the negative charge on the oxygen and stabilises the phenoxide ion. Hence, phenol is more acidic than any other alcohol.