The decreasing order of acidity is
p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol.
Nitrophenol is more acidic than phenol. Nitro group is electron withdrawing group. It stabilizes the phenoixde ion by dispersing negative charge and facilitates the release of H+ ions from phenol. This acid strengthening effect is more when a substitutnt is present in o- or p- position than in m- position to the phenolic group.