In the present study, experiments were carried out in a single-cylinder downsized SI engine with different rates of oxygen (15% to 27% by volume in the total mixture of intake gases except fuel) and equivalence ratios (from 0.45 to 1). Therefore, the oxygen volume fraction is due to oxygen enrichment or nitrogen dilution. The study of the impact of controlling oxygen concentration on the combustion characteristics and emissions was performed at 1400 rpm, at several loads (Indicated Mean Effective Pressure (IMEP) from 400 to 1000 kPa). For each operation point, the spark advance and the intake pressure were adjusted simultaneously in order to maintain the load and obtain a minimum value of indicated Specific Fuel Consumption (SFC).The effect of oxygen concentration on the engine combustion characteristics was simulated by using the commercial software AMESim, with the combustion model developed by IFP-EN, and an adapted algorithm was used to avoid residual gas calibration. The in-cylinder pressures are calibrated with experimental data by adapted the integral length scale and tumble number values. By implementing a correlation for the laminar burning velocity, the in-cylinder pressures were perfectly calibrated with a maximum pressure relative error less than 2% for almost all the operation points.