Mononuclear Ru-based water oxidation catalysts (WOCs) constitute an important class of WOCs for water splitting. This work constitutes a systematic study of Ru-O2 complexes of mononuclear ruthenium WOCs, with a focus on the thermodynamics of water-assisted O2 release in various electronic states and conformations. Our extensive DFT study reveals several factors that affect the O2 release thermodynamics: (1) steric effect from the ligand sphere of Ru; (2) trans effect of ligands trans to O2; (3) oxygen cis coordinating effect; (4) carbon coordinating effect; and (5) Ru coordination strength. Some of these effects could selectively stabilize/destabilize some states/conformations of the Ru-O2 complexes relative to Ru-OH2 complexes, and affect thereby the O2 release thermodynamics. The identification and rationalization of factors for O2 release thermodynamics, as in this work, could be helpful toward a better understanding of this final step of the ruthenium-catalyzed water oxidation.