Complex dielectric function of very thin ZnO films prepared with different oxygen concentration during deposition has been studied in 220-1700 nm and 2-25 mu m spectral range. Variation of oxygen concentration in deposition chamber is intended to stabilize film properties by compensating oxygen deficiency. The films obtained by magnetron sputtering with thicknesses below 100 nm and optical axis perpendicular to sample surface were measured in standard ellipsometry configurations using rotating compensator ellipsometers. Dielectric function of the films obtained by fitting the experimental data using oscillator model exhibits narrowing and band edge oscillator center energy shifts from 3.178 to 3.355 eV for samples prepared with 0.5% and 1-2% oxygen, respectively. Results suggest similar to 2% as the optimal oxygen concentration for preparation of optically stable films. Infrared dielectric function of the film prepared with 2% oxygen is also obtained. (C) 2014 Elsevier B.V. All rights reserved.