Nitro group of phenol produces -I and -R effect. Because of these two effects `-NO_(2)` group is electron withdrawing in nature. So, the electron density in the O-H bond of p-nitrophenol decreases relative to the O-H bond of phenol.
The decrease in electron density of the O-H bond of p-nitrophenol, the polarity of O-H bond is decreases and in turn make it more acidic than phenol.
