A method for estimating the oxygen concentration in an internal combustion engine that includes, but is not limited to an intake manifold, an exhaust manifold, an EGR system, a throttle valve, an air mass sensor for measuring a fresh air flow ({dot over (m)}thr) entering the intake manifold through the throttle valve, and cylinders. The method includes, but is not limited to the steps of estimating the total gas flow ({dot over (m)}o) entering the cylinders, calculating the EGR gas flow ({dot over (m)}egr), calculating the air fraction (f—air—em) of the gas flowing in the exhaust manifold, calculating the air mass (mim—air) entering the cylinders based on the air fraction (f—air—em) in the exhaust manifold, on the total gas flow ({dot over (m)}o) entering the cylinders, on the EGR gas flow ({dot over (m)}egr) and on the fresh air flow ({dot over (m)}thr), calculating the total mass (mim) in the intake manifold based on the fresh air flow ({dot over (m)}thr), on the EGR gas flow ({dot over (m)}egr) and on the total gas flow ({dot over (m)}o) entering the cylinders, calculating the air fraction (fair—im) in the intake manifold based on the air mass (mim—air) entering the cylinders and the total mass (mim) in the intake manifold, and calculating the oxygen mass concentration ([O2]m—im) in the intake manifold based on the air fraction (fair—im) in the intake manifold.