A method for estimating the oxygen concentration in an internal combustion engine comprising an intake manifold, an exhaust manifold, an EGR system, a throttle valve, an air mass sensor for measuring a fresh air flow ( ṁ thr) entering the intake manifold through the throttle valve, a plurality of cylinders, the method being: - estimating the total gas flow ( ṁ o) entering the cylinders; - calculating the EGR gas flow ( ṁ 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 ( ṁ o) entering the cylinders, on the EGR gas flow ( ṁ egr) and on the fresh air flow ( ṁ thr); - calculating the total mass (m im) in the intake manifold based on the fresh air flow ( ṁ thr), on the EGR gas flow ( ṁ egr) and on the total gas flow ( ṁ o) entering the cylinders; - calculating the air fraction (f _air_im) in the intake manifold based on the air mass (m im_air) entering the cylinders and the total mass (m im) in the intake manifold, and - calculating the oxygen mass concentration ([O 2] m_im) in the intake manifold based on the air fraction (f_air_im) in the intake manifold; wherein the estimation of the total gas flow ( ṁ o) entering the cylinders and of the EGR gas flow ( ṁ egr) is carried out by: - determining an estimated pressure (p im) and a measured pressure (p im-sens) in the intake manifold, and - estimating a theoretical total gas flow ( ṁ oTH) entering the cylinders, wherein the estimation of the EGR gas flow (ṁ egr) is carried out by the following equation: m ̇ egr m ̇ o − m ̇ thr + P p im_sens − p im where P is a predetermined proportional factor, and wherein the total gas flow ( ṁ o) is the theoretical gas flow entering the cylinders ( ṁ oTH), wherein the method further comprises the step of determining an estimated temperature (T im) in the intake manifold and wherein the estimated pressure (p im) in the intake manifold is calculated according to the following equation: P im = R im m im T im V im where Vim is a constant representative of the geometrical volume of the intake manifold, and Rim is the constant R of the gas, and wherein the method further comprises the steps of measuring a temperature (T im_sens) in the intake manifold and wherein the estimated temperature (Tim) in the intake manifold is calculated according to the following equations: T im_ideal = P im_sens V im R im m im { T im_obs = L. P. F T im T im = T im_ideal + P. I. T im_sens − T im_obs where V im is a constant representative of the geometrical volume of the intake manifold, R im is the constant R of the gas, L.P.F is a predetermined low pass filter, T im_obs is an observed temperature value generated by a low pass filter model taking into account the temperature sensor time constant and P.I. is a proportional integral controller. - estimating the total gas flow ( ṁ o) entering the cylinders; - calculating the EGR gas flow ( ṁ 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 ( ṁ o) entering the cylinders, on the EGR gas flow ( ṁ egr) and on the fresh air flow ( ṁ thr); - calculating the total mass (m im) in the intake manifold based on the fresh air flow ( ṁ thr), on the EGR gas flow ( ṁ egr) and on the total gas flow ( ṁ o) entering the cylinders; - calculating the air fraction (f _air_im) in the intake manifold based on the air mass (m im_air) entering the cylinders and the total mass (m im) in the intake manifold, and - calculating the oxygen mass concentration ([O 2] m_im) in the intake manifold based on the air fraction (f_air_im) in the intake manifold; - determining an estimated pressure (p im) and a measured pressure (p im-sens) in the intake manifold, and - estimating a theoretical total gas flow ( ṁ oTH) entering the cylinders,