We report on the optical humidity sensitivity of nickel oxide (NiO) films prepared by plasma oxidation of nickel-carbon composite films. The plasma-oxidized NiO films showed humidity-sensitive absorbance change at room temperature. The optical absorbance change was strongest at wavelengths around 560 nm. The reversible absorbance change over a wide humidity range of 0-90% RH and the response time within several minutes were obtained with the plasma-oxidized NiO films which is a potential candidate for optical humidity detection.