Phenols are much more acidic than alcohols, due to the stabilisation of phenoxide ion by resonance.
Phenoxide ion is stabilized due to following resonating structures:
Ortho nitrophenol is most acidic because in it $- NO _{2}$ electron attracting group is attached on ortho position which helps in stabilising of negative charge on the oxygen of phenoxide ion.Hence, due to this reason acidic character of phenol is increased, while on attachment of $- CH _{3}$ group (electron donating group) acidic strength of phenol is decreased in cresol due to destabilisation of phenoxide ion.
