As in ortho-Nitrophenol H of −OH is already in Intra-Moleculer H Bond and become stable so it will not participate in H bonding with H2O by breaking this H bonding. That is why solubility of Ortho-Nitrophenol is less in water.
para- and meta- Nitrophenols both shows inter-Moleculer H bonding with H2O. That is why both are more soluble in water.