OH is a strong field or weak field ligand? 

The spectrochemical series indicating weak to strong field ligand is as follows:

I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < N₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < CH₃CN < py  < NH₃ < en  < bipy < phen  < NO₂⁻ < PPh₃ < CN⁻ < CO

Normally for the first series of transition elements left from the H₂O  all the ligands are weak in nature while right from it are strong. 

So, OH is a weak ligand