$-\mathrm{N}{\mathrm{O}}_2$ is an electron attracting group whereas $-\mathrm{C}{\mathrm{H}}_3$ is an electron releasing group. An electron attracting substituent tends to disperse the negative charge of phenoxide ion and thus makes it more stable. This in turn increases the acidity of phenol. The substituent in Para-position is more effective than in meta-position as the former involves a resonating structure bearing negative charge on the carbon attached to electron with drawing substituent.
An electron releasing substituent tends to increase the negative charge of peroxide ion and thus makes it more unstable. This, in turn, decreases the acid strength of phenol.