Ortho isomer shows intramolecular hydrogen bonding , in which the H-bonding takes place in a single molecule not with the adjacent molecule. And in that case the molecules exists as discrete molecules or particles only because they are not attached or attracted by each other.
Thus their boiling point decreases and they are volatile.
But in case of para-nitrophenol the intermolecular hydrogen bonding takes place which allows the adjacent molecules to join and remain in an associated form. And due to this the boiling point of para isomer increases.
The Unconditional Guru 