The inhibitory effect of molecular oxygen on chemical oscillations in a semiclosed Belousov-Zhabotinsky system is investigated. Comparison of experimental results obtained from anaerobic and aerobic reactions shows that the oxygen-induced increase of oscillation periods is most pronounced at low concentrations of sulfuric acid or bromate and at high concentrations of malonic acid. Under the given experimental conditions, oxygen inhibition is negligible for [H2SO4] > 0.35 M, [BrO3-] > 0.15 M, and [CH2(COOH)(2)] < 0.07 M. Based on the classical Oregonator, we propose an extended model that accounts for the observed concentration dependencies. This model treats oxygen as a dynamic variable and is capable of describing oxygen oscillations as well as the striking increase in oxygen concentration for conditions in which oscillations are fully suppressed. The rate equations of the extended Oregonator model are presented in a dimensionless form and are, for anaerobic conditions, in exact correspondence to the classical model.