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Native defects in semiconductors play an important role in their optoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-type oxide semiconductors and its conductivity is believed to be controlled primarily by Ni-vacancy acceptors. Herein, we present a systematic study comparing the optoelectronic properties of stoichiometric NiO, oxygen-rich NiO with Ni vacancies (NiO:VNi), and Ni-rich NiO with O vacancies (NiO:VO). The optical properties were obtained by spectroscopic ellipsometry, while valence band spectra were probed by high-resolution x-ray photoelectron spectroscopy. The experimental results are directly compared to first-principles density functional theory + U calculations. Computational results confirm that gap states are present in both NiO systems with vacancies. Gap states in NiO:Vo are predominantly Ni 3d states, while those in NiO:VNi are composed of both Ni 3d and O 2p states. The absorption spectra of the NiO:VNi sample show significant defect-induced features below 3.0 eV compared to NiO and NiO:VO samples. The increase in sub-gap absorptions in NiO:VNi can be attributed to gap states observed in the electronic density of states. The relation between native vacancy defects and electronic and optical properties of NiO are demonstrated, showing that at similar vacancy concentration, the optical constants of NiO:VNi deviate significantly from those of NiO:VO. Our experimental and computational results reveal that although VNi are effective acceptors in NiO, they also degrade the visible transparency of the material. Hence, for transparent optoelectronic device applications, an optimization of native VNi defects with extrinsic doping is required to simultaneously enhance p-type conductivity and transparency.