© 2018 American Chemical Society. One of the major limitations of oxide semiconductors technology is the lack of proper p-type materials to enable devices such as pn junctions, light-emitting diodes, and photodetectors. This limitation has resulted in an increased research focus on these materials. In this work, p-type NiOx thin films with tunable optical and electrical properties as well as its dependence with oxygen pressure during pulsed laser deposition are demonstrated. The control of NiOx films resistivity ranged from ∼109 to ∼102 ω cm, showing a p-type behavior with Eg tuning from 3.4 to 3.9 eV. Chemical composition and the resulting band diagrams are also discussed. The all-oxide NiOx-Ga2O3 pn junction showed very low leakage current, an ideality factor of ∼2, 105 on/off ratio, and 0.6 V built-in potential. Its J-V temperature dependence is also analyzed. C-V measurements demonstrate diodes with a carrier concentration of 1015 cm-3 for the Ga2O3 layer, which is fully depleted. These results show a stable, promising diode, attractive for future photoelectronic devices.