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Solution-processed nickel oxide (sNiO) is known to be an excellent charge-selective interlayer in optoelectronic devices. Its beneficial properties can be further enhanced by an oxygen plasma (OP) treatment. In order to elucidate the mechanism behind this improvement, we use infrared transmission and X-ray photoelectron spectroscopies to probe the bulk and surface properties of the sNiO. We find that increasing the annealing temperature of the sNiO not only increases the structural order of the material, but also reduces the concentration of nickel hydroxide species present in the bulk and on the surface of the film. This results in a decrease of the work function, while an additional OP treatment raises the work function to between 5.5 eV and 5.6 eV. For all annealing temperatures investigated, the consequences of the OP treatment are identified as reactions of both NiO and β-Ni(OH)₂ to form thin β-NiOOH phases in the first atomic layers. Our results emphasize the importance of understanding the correlation between the preparation and resulting properties of sNiO layers and provides further insight into the interpretation of interface properties of NiO.