Phenol is less acidic than __________.

In o-nitrophenol, nitro group is present at ortho position. Presence of electron withdrawing group at ortho position increases the acidic strength. 

On the other hand, in o-methylphenol and in o-methoxyphenol electron releasing group (-CH₃ or OCH₃), at ortho or para positions of phenol decreases the acidic strength of phenols. So, phenol is less acidic than o-nitrophenol.