The authors study the thermal oxidation of nickel thin films (50nm) fabricated by conventional thermal evaporation, resulting from annealing in air at 300, 325, 350, 400, and 700°C. The characterization is performed by x-ray diffraction, Raman spectroscopy, superconducting quantum interference device magnetometry, and scanning electron microscopy. These techniques show that the oxidation increases with annealing temperature. The formation of granular films of coexisting Ni and NiO is confirmed after annealing at 400°C. The magnetic measurements indicate coexisting ferromagnetism and antiferromagnetism, corresponding to Ni and NiO contributions. The magnetic hysteresis loops reveal exchange bias in the samples annealed at 235, 350, and 400°C due to the competition between the exchange interactions at the Ni/NiO interfaces.