The structural, electronic, and dynamic properties of hypothetical gold(II) oxide (AuO) are studied theoretically, at atmospheric and elevated pressures, with the use of hybrid density functional theory. At p = 1 atm, hypothetical AuO (metastable with respect to the elements) is predicted to crystallize in a new structure type, unique among the late-transition-metal monoxides, with disproportionation of the Au ions to Au(I/III) and featuring aurophilic interactions. Under pressure, familiar structure types are stabilized: a semiconducting AgO-type structure at ∼2.5 GPa and, with a further increase of the pressure up to ∼80 GPa, an AuSO4-type structure containing Au2 pairs. Finally, above 105 GPa, distorted NaCl- and CsCl-type Au(II)O structures dominate, and metallization is predicted at 329 GPa.