$o$-nitrophenol is more volatile than $p$-nitrophenol. In para nitrophenol, intermolecular hydrogen bonding is present. This intermolecular hydrogen bonding causes the association of molecules. In $o$-nitrophenol an intramolecular hydrogen bonding between the partial positively charged $(\delta+)$ hydrogen of phenolic group and between the partial negatively charged ( $\delta-)$ oxygen of nitro group. So, it involves only a single molecule, and the boiling point is lesser and so the compound is highly volatile.In $p$-nitrophenol, there exists intermolecular hydrogen bonding. Here, positively charged $(\delta+)$ hydrogen in the phenolic group of one molecule and the partial negatively charged $(\delta-)$ oxygen in nitro group of another molecule. So, the hydrogen bonding involves multiple molecules. The boiling point is higher and compound is less volatile compared to $o$-nitrophenol.