Location of the oxygen dopant in the high temperature superconductor HgBa2CuO4+δ from 199Hg NMR

•Oxygen is the primary dopant that leads to superconductivity in HgBa2CuO4+δ.•A technique for oxygen isotope exchange and annealing to change doping is described.•The concentration of dopant oxygen is determined through nuclear magnetic resonance.•Results imply that dopant oxygen atoms are randomly distributed in this cuprate.