Ab initio molecular dynamics simulations with an adaptive biasing potential are carried out to study the reaction path in mononuclear Ru catalysts for water oxidation of the type (Ar)Ru(X)(bpy)]+ with different aromatic ligands (Ar). The critical step of the O-O bond formation in the catalytic cycle starting from the (Ar)Ru(O)(bpy)]2+ intermediate is analyzed in detail. It is shown that an explicit inclusion of the solvent environment is essential for a realistic description of the reaction path. Clear evidence is presented for a concerted reaction in which the O-O bond formation is quickly followed by a proton transfer leading to a Ru-OOH intermediate and a hydronium ion. An alternative path in which the approaching water first coordinates to the metal centre is also investigated, and it is found to induce a structural instability of the catalyst with the breaking of the aromatic ligand coordination bond.