During steam distillation, it is the lower boiling compound which distills out first. Between ortho- and para-nitrophenol it is the ortho- isomer which will be steam volatile since it has a lower boiling point.
The difference in boiling point betweenthe two isomers can be understood based on the structural difference. In ortho-isomer intramolecular hydrogen bonding takes place while in the para-isomer, intermolecular hydrogen bonding takes place.As a result of the strong forces operating between the molecules of p-isomer, the boiling point is higher and it is not steam volatile.