We perform extensive density-functional theory total-energy calculations and ab initio molecular-dynamics simulations to evaluate the structure, stability, and reactivity of oxygen- and hydroxyl-decorated InP(001) surfaces for photoelectrochemical water cleavage. Surface oxygen is adsorbed in one of two primary local bond topologies: In–O–P and In–O–In. We show that the chemical activity of the oxygen-decorated surface toward water dissociation can be connected to the local oxygen bond topology, with In–O–In bridges promoting spontaneous water dissociation. Surface hydroxyl groups tend to form either In–OH–In bridges, though the second of the two In–OH bonds is easily broken. Dynamics simulations of the full water–semiconductor interface show surface proton transfer when the surface is hydroxylated, facilitated by strong hydrogen bonding between atop OH groups and with interfacial water molecules. Implications for understanding the reaction dynamics at InP(001)–water interfaces are discussed.