Transient emission peaks have become an important fraction of the total emissions during the standardized test cycles for passenger car Diesel engines. This paper is concerned with their reduction, in particular of nitric oxides (NO X) and particulate matter (PM) emissions, by online receding horizon optimal control. It is based on former works [ 1, 2 ] in which alternative target quantities for engine control were proposed, namely in-cylinder oxygen concentrations before (O 2,BC) and after combustion (O 2,AC). The actual work is concerned with testing an in-cylinder oxygen concentrations based control in simulation as well as by a real-time implementation on a turbocharged common rail passenger car production Diesel engine. The promising results confirm the choice of these concentrations as sensible control references and the feasibility of a real-time use in a model predictive control implementation. Copyright © 2010 SAE International.