Understanding the response of plant nitrogen (N) and carbon (C) economies in oilseed rape, as well as their role in defining phenotypic plasticity, is necessary for designing new strategies to optimize plant and canopy C assimilation to improve potential yield. This paper aims to elucidate the extent to which the interaction between N supply and plant population density alters N distribution in oilseed rape plant (Brassica napus L.) and whether this interaction changes plant investment in leaf area or leaf mass per area. Spring oilseed rape was grown at two rates of N supply (50 and 150 kg N·ha−1) and two plant population densities (50 and 150 plants·m−2). Photosynthesis, leaf area, leaf biomass, and N content of selected leaves were measured at 20% of flowers on main raceme open. The interaction between N supply and plant population density altered leaf N content per area, which is the main determinant of photosynthesis. This interaction also affected leaf mass per area, while N supply determined N content per unit leaf mass. These results suggest that the interaction between N supply and population density affects both nitrogen distribution and leaf mass per area, which could have important implications for light distribution and, therefore, for C assimilation at the plant level.