Presence of electron attracting or withdrawing groups e.g NO2,X,CN,CHO,COOH on the benzene ring increases the acidity of phenol as it enables the ring to draw more electrons from the phenoxy oxygen and thus releasing the proton easily.Further,the particular effect is more when the substituent is present on o or p position than in m position to the phenolic group(as additional contributing structure is obtained by the dispersal of negative charge over oxygen atom of NO2 group also in o and p nitrophenoxide but not in m phenoxide).A m nitro group  cannot enter into resonance with the hydroxyl group but can exert -Ieffect for this position.hence  is stronger than phenol.Out of o and p nitrophenol , p nitrophenol is slightly stronger than o nitrophenol as o nitrophenol  is a bit stabilised by intramolecular hydrogen bonding,thus having a reduced tendency to release a proton.
						

							
while the previous answer gives reasons as to why mVspVso principle works... within o- phenol and p-phenol it is not really possible to differentiate logically unless other condition is present which can give us additional information. like a solvent which might inhibit one of them or something like that.
						

							
Out of o and p nitrophenol , p nitrophenol is slightly stronger than o nitrophenol as o nitrophenol  is a bit stabilised by intramolecular hydrogen bonding,thus having a reduced tendency to release a proton.  i think this shud be d reason .pls someone tell me if i am wrong.