Oxyhydroxyapatite and oxyapatite impact the fields of medicine, energy generation, sensing, and photonics. While oxyapatite may be stabilized with trivalent cations, oxyhydroxyapatite is inherently metastable and represents a pathway from hydroxyapatite to oxyapatite that subsequently decomposes to tricalcium phosphate and tetracalcium phosphate. Hydroxyl ions in hydroxyapatite are most mobile and upon exposure to high temperatures the first to leave the lattice to form oxyhydroxyapatite. There have been several attempts to obtain oxyapatite by heating, but its metastability and high dehydroxylation levels promote decomposition towards tricalcium and tetracalcium phosphates. However, upon cooling the hygroscopic nature readily leads to the return of hydroxyl ions into the lattice. Phase-pure oxyapatite at room temperature has not been successfully obtained yet. The possible microstructures formed by dehydroxylation during different thermal processes will be discussed with reference to oxyhydroxyapatite solid solutions or mechanical mixtures of hydroxyaptite and oxyapatite. The largest impediment to active research in this field is the non-availability of a userfriendly approach to measure the concentration of hydroxyl ions. Different approaches will be discussed. The properties of oxyapatite are ill-defined yet, but a comment will be made on what can be expected and how this may lead to a range of different applications.