Question

why OH group is ortho para directing group?

Answer 1

Because the -OH group is a strongly activating group (see Electrophilic aromatic directing groups). 

The -OH group tends to be electron-donating because the extra electrons around the oxygen atom can be shared with the aromatic benzene ring. Thus, it contributes resonance structures that can be seen below.



As you can see, the resonance structure of the phenol molecule includes 2, 4, and 6 positions (or ortho/para positions) where those positions have a partial negative charge. This opens up these positions for possible attack by an electrophile, through the process known as electrophilic aromatic substitution.

Answer 2

since OH is +R/+M group thus provides e- to the benzene ring thus creates +ve charge on meta position and -ve charge on ortho and para position thus it is ortho para directing group...