Zinc oxide (ZnO) thin films annealed at different temperatures were studied with photoluminescence (PL), electrical resistivity, Hall mobility, and 1/f noise spectroscopy. Relatively high electrical conductivity and carrier concentration in sample annealed at 400 °C suggested the presence of ZnO interstitials. Rapid reduction in electrical conductivity and carrier concentration upon increasing the annealing temperature suggested that ZnO interstitials could be eliminated by high temperature annealing. Presence of G-R noise in sample annealed at 400 °C indicated high level of electron trapping activities. Density of Zn vacancies acting as electron traps was estimated by Lorentzian fitting on the G-R noise. PL spectra exhibiting dominant green emission in all samples suggested the presence of Zn vacancies in high concentration. Yellow-orange emission in PL in samples annealed at 600 °C and below indicated the presence of O interstitials, while the same emission in samples annealed at higher temperatures were ascribed to Si impurities diffused from the substrate. Sharp reduction in mobility and surge in Hooge’s parameter in sample annealed at 700 °C implied high level of electron scattering due to large extrinsic Si impurities. Gradual rise in green-yellow emission and electron concentration as annealing temperature increased from 500 to 700 °C were ascribed to the gradual formation of O vacancies.