In the study we used a number of high level theoretical methods to calculate the O-H bond dissociation energies (BDEs) as well as α and remote para substituent effects on them. We found that only G3 and CBS-Q methods can be used to calculate the absolute O-H BDEs. Other methods including B3LYP, MP2, and CCSD(T), either open-shelled or close-shelled, significantly underestimate the O-H BDEs. To be even worse, except for G3 and CBS-Q, the other theoretical methods cannot accurately predict the α substituent effects on O-H BDEs either. Methods including UMP2, ROMP2, and UCCSD(T) may even provide qualitatively erratic α substituent effects. Using the G3 and CBS-Q results, we found that the α substituent effect on the O-H BDEs is usually much larger than that on the C-H or N-H BDEs. Both the π donors and acceptors reduce the O-H BDEs because of the conjugation and hyperconjugation between the substituent and the radical center. Polyfluorinated alkyl groups increase the O-H BDEs because of the inductive electron-withdrawing effect. In comparison, an electron-withdrawing para substituent increases the O-H BDE of phenol, whereas an electron-donating group reduces it. The calculated ρ + value for the O-H BDEs of phenols is about 4-5kcal/mol. Compared to it, the experimentally determined ρ + value is significantly larger because of the solvent effect. Furthermore, the ρ + values for the O-Y BDEs of 4-X-C 6 H 4 -O-Y decrease in the order O-CH 3 >O-H>O-OCH 3 >O-OH>O-NO.