The most straightforward measure of the strength of a bond is the energy needed to break it. Estimates of such bond energies are usually obtained from thermo chemical cycles using Hess' Law and are called bond enthalpies. A bond dissociation enthalpy is the enthalpy change occupied in breaking the bond to one atom in a molecule, and in a diatomic is by definition same to the bond enthalpy. Thus the enthalpy of dissociation of O2 places directly the (double) bond enthalpy B(O=O).
When a molecule occupies several equal bonds, the enthalpy required to dissociate them successively is not the exact. Instead of dealing with individual bond dissociation energies, it is normal to describe the mean bond enthalpy. Thus B (O-H) is describe as half the enthalpy change in the process.