o-nitrophenol is more acidic than o-cresol. Acidic character of alcohols depends upon the polarity of $\mathrm{O}-\mathrm{H}$ bond. In o-nitrophenol due to $-\mathrm{I}$ and $-\mathrm{R}$ effect of nitro-group electron density decreases in $\mathrm{O}-\mathrm{H}$ bond and polarity increases.
While in o-cresol, due to $+\mathrm{I}$ effect of $-\mathrm{CH}_3$ group electron density of $\mathrm{O}-\mathrm{H}$ bond increases. Therefore, $\mathrm{O}-\mathrm{H}$ bond becomes less polar. Hence, o-nitrophenol is more acidic than o-cresol.