Dual-loop exhaust gas recirculation (EGR) systems can provide control authorities to adjust the engine in-cylinder gas conditions. The significant transport delays in the EGR air-paths can affect the performance of some simple oxygen concentration dynamic models under transient operating conditions. In this paper, a physics-based dual-loop EGR air-path oxygen concentration model considering the time-varying transport delays is developed and a method to calculate the delay time based on the continuity of fluid velocity is presented. Simulation studies with a high-fidelity, one-dimensional, computational GT-Power engine model and experimental validations on a Diesel engine test bench show that the developed model performs well under all tested operating conditions while the comparison models cannot capture the oxygen concentration dynamics under some transient operating conditions. (C) 2014 Elsevier Ltd. All rights reserved.