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 (δ+) hydrogen of phenolic group and between the partial negatively charged ( δ−) 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 (δ+) hydrogen in the phenolic group of one molecule and the partial negatively charged (δ−) 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.