Octacalcium phosphate (OCP) has been suggested as a precursor of biological apatite in bone, dentin, and cementum because its existence explains the nonstoichiometry of apatite crystals in their compositions. Synthetic inorganic calcium phosphate compounds have been used clinically to fill bone defects, and sintered hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP), bone substitute materials, are known to be osteoconductive, with beta-TCP also being bioresorbable. Nonsintered synthetic OCP has been shown to enhance bone regeneration accompanied by conversion into hydrolyzed apatitic products in situ and biodegradation. The surfaces of the OCP implant and the converted apatite seem to be continuously exposed to biological constituents, such as extracellular matrices, inorganic biominerals, and cellular components. This article reviews the surface reaction of OCP implants and the biological responses, such as experimentally stimulated bone formation on synthetic OCP, the mechanism of OCP hydrolysis into apatite, and the adsorption of biomolecules onto OCP and the converted apatite, of particular interest in reactive bone induction with synthetic OCP implants.