Water binding to the Mn4O5Ca cluster of the oxygen-evolving complex (OEC) of Photosystem II (PSII) poised in the S2 state was studied via H2 17O- and 2H2O-labeling and high-field electron paramagnetic resonance (EPR) spectroscopy. Hyperfine couplings of coordinating 17O (I = 5/2) nuclei were detected using W-band (94 GHz) electron–electron double resonance (ELDOR) detected NMR and Davies/Mims electron–nuclear double resonance (ENDOR) techniques. Universal 15N (I = 1/2) labeling was employed to clearly discriminate the 17O hyperfine couplings that overlap with 14N (I = 1) signals from the D1-His332 ligand of the OEC ( Stich Biochemistry 2011, 50 (34), 7390−7404 ). Three classes of 17O nuclei were identified: (i) one μ-oxo bridge; (ii) a terminal Mn–OH/OH2 ligand; and (iii) Mn/Ca–H2O ligand(s). These assignments are based on 17O model complex data, on comparison to the recent 1.9 Å resolution PSII crystal structure ( Umena Nature 2011, 473, 55−60 ), on NH3 perturbation of the 17O signal envelope and density functional theory calculations. The relative orientation of the putative 17O μ-oxo bridge hyperfine tensor to the 14N(15N) hyperfine tensor of the D1-His332 ligand suggests that the exchangeable μ-oxo bridge links the outer Mn to the Mn3O3Ca open-cuboidal unit (O4 and O5 in the Umena et al. structure). Comparison to literature data favors the Ca-linked O5 oxygen over the alternative assignment to O4. All 17O signals were seen even after very short (≤15 s) incubations in H2 17O suggesting that all exchange sites identified could represent bound substrate in the S1 state including the μ-oxo bridge. 1H/2H (I = 1/2, 1) ENDOR data performed at Q- (34 GHz) and W-bands complement the above findings. The relatively small 1H/2H couplings observed require that all the μ-oxo bridges of the Mn4O5Ca cluster are deprotonated in the S2 state. Together, these results further limit the possible substrate water-binding sites and modes within the OEC. This information restricts the number of possible reaction pathways for O–O bond formation, supporting an oxo/oxyl coupling mechanism in S4.