The interface at NiO and GaAs heterojunction is found to have claudetite phase of As₂O₃ with monoclinic structure, which increases with oxygen (O₂) partial pressure. It is determined that rough interface, caused by recrystallization of GaAs surface along [111] direction, is not the main governing factor for the observed change in growth direction for NiO epitaxial layer from [111] to [001] direction with increase in O₂ partial pressure. The out-of-plane and in-plane epitaxial relationships of [111] oriented NiO layer with respect to substrate are [111]_NiO || [001]_GaAs, [-1-12]NiO || [-1-10]_GaAs and [-110]_NiO ||[-110]_GaAs. The interfacial structure of NiO/GaAs heterojunction thus determined can have implications into the characteristics of optoelectronic devices based on this heterojunction.