By combining X-ray crystallography, first-principles density functional theory calculations, and solid-state nuclear magnetic resonance spectroscopy, we have refined the crystal structure of octacalcium phosphate (OCP), reassigned its P-31 NMR spectrum, and identified an extended hydrogen-bonding network that we propose is critical to the structural stability of OCP. Analogous water networks may be related to the critical role of the hydration state in determining the mechanical properties of bone, as OCP has long been proposed as a precursor phase in bone mineral formation. The approach that we have taken in this paper is broadly applicable to the characterization of crystalline materials in general, but particularly to those incorporating hydrogen that cannot be fully characterized using diffraction techniques.