Where will electrophilic substitution take place in meta-nitrophenol (3-nitrophenol) ? The two mesomeric effects seem to compete against each other.
Electrophilic substitution can happen at only that place where electrons are present which can be donated/shared with some other electron-deficient species. In the resonance structures of m-nitrophenol shown above , there is a negative charge on hydrogen or carbon(because charge keeps on shifting between carbon and Hydrogen) and they are not that electronegative to handle the negative charge . So they want to stabilize themselves by giving off their electrons to electron deficient species by letting electrophilic substitution happen. You can see lone pair (negative charge ) of OH group is not delocalised by NO2 group if it is present in meta position . Although NO2 group is still in Conjugation with benzene ring but for OH group, NO2 can show nothing but only -Inductive Effect. So , now after making changes in my answer , In my opinion , electrophilic substitution will happen on left side of the molecule at ortho position. On the right side at ortho position , electron density is less as compared to the left side because NO2 is showing -I effect making conditions less suitable for electrophilic substitution to happen.