This paper examines the oxygen evolving reaction of water splitting under practical conditions which simulate those encountered in photo-initiated or electrochemical water oxidation processes. Most of the over-potentials are due to electrochemical processes at the anode, where oxygen evolution occurs. This paper investigates the oxygen evolving half cells for different complete systems including photoelectrochemical, photo-catalytic and electro-catalytic water splitting. An electrochemical model is developed to evaluate the over-potential losses in the oxygen evolving reaction and the effects of key parameters are analyzed. The transient diffusion of hydroxide ions through the membrane and bulk electrolyte are modeled and simulated for improved system operation. The results of the thermodynamic and electrochemical analyses show that for each water splitting configuration, there are optimal values of the operating parameters such as electrolyte concentration, current density, and membrane-electrode distance. The operating criteria of key parameters and the optimal working region of the oxygen evolving reactor are examined for assessment and optimization of a complete water splitting system. The analysis of the oxygen evolving reaction is performed for three variations of ruthenium based supramolecular complexes and molybdenum-oxo catalysts for catalytic hydrogen production. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.