In meta hydroxy benzaldehyde which position (w.r.t hydroxy group) is more prone to electrophilic attack? In meta hydroxy benzaldehyde which position (w.r.t hydroxy group) is more prone to electrophilic attack ? 

My approach:I considered hydroxy group as the stronger +M group.So my guess was para position w.r.t hydroxy group.But that's not the correct answer.

The answer is actually given as ortho w.r.t hydroxy and para w.r.t -CHO group!!
This is known as the ortho effect. According to it if an ortho/para and meta directing group are meta to each other, then the incoming electrophile prefers to come on ortho of meta directing group.

See the reference.