In an O-H bond, will polarity increase/decrease if some of the electron density on the oxygen atom is reduced?
The polarity on the oxygen atom will decrease. This is because of the sp2 hybridised carbon atom which is highly electronegative. This will result in a decreased polarity in the OH bond and hence it will disassociate more easily in solvents such as water. One example of this effect is the increased acidity of phenol compared to most alcohols. This is because the OH is bonded to sp2 hybridised carbon in phenol and hence will disassociate in water easily when compared to normal alcohols.