NiO and ZnO are a pair of p- and n-type wide band gap metal oxide semiconductors that forms a partially soluble system with each other. Due to these properties, they are considered a potential semiconductor pair for the construction of a p-n heterojunction that remains thermodynamically stable under high-temperature applications. To this end, an in-depth understanding of the properties of the available interface arrangements, as well as of the effects of the formation of the mixed phases on the properties of the semiconductors is necessary. This study seeks to investigate these aspects by a theoretical approach, utilizing DFT to model the properties of the possible interfaces, as well as the behavior of the mixed phase systems at different concentrations of solute ions. The bulk properties of nickel substituted ZnO (Ni:ZnO) and zinc substituted NiO (Zn:NiO) were investigated utilizing a hybrid functional approach. Both systems were found to exhibit an increase in the VBM energy level relative to the pure materials, with maxima of 0.39 eV and 0.26 eV, respectively. The VBM change with solute ion concentrations has been found to reach saturation at 3.70% and 4.69%, again respectively. The VBM shifts of Ni:ZnO and Zn:NiO are found to be caused by the formation of a new valence band due to localized impurity states, and the formation of cubic ZnO bonding states, respectively. For both mixed phase materials, a reduction of the band gap width is observed: The Ni:ZnO band gap is reduced by 0.3 eV, while the Zn:NiO band gap changes linearly with concentration throughout the experimental concentration range, with a band gap reduction of 0.8 eV at 31.25% zinc concentration. The structural, energetic and electronic properties of the NiO-ZnO interfaces formed from pairs of low-Miller index surfaces were investigated utilizing a GGA+U functional approach. The polar-polar interfaces were found to exhibit the most favorable interface formation energies, for either strain distribution considered. The natural band offsets were found to all form a type II heterojunction with valence band offset values ranging from 0.73 to 1.84 eV. The majority of the considered interface arrangements exhibit considerable interface states: Conduction band interface states are the most prevalent, formed from nickel 3d-orbitals. The results indicate that the prevalence of these interface states, and the general NiO-ZnO heterojunction properties, may be manipulated to a considerable extent by choosing a suitable interface, and the growth conditions under which it is formed.