Understanding the response of the number of seeds and seed weight to the availability of assimilates is crucial for designing breeding strategies aimed to increase seed and oil yield in oilseed rape. This study aims to answer the questions: i) do seed number and seed weight in oilseed rape differ in their plasticity in response to the availability of assimilates at flowering? and ii) how sensitive are oil and protein concentrations to the availability of assimilates during flowering? A spring oilseed rape hybrid was sown in two field experiments and the treatments were combinations of i) two plant densities and ii) shading or no shading between the beginning and end of flowering. Seed yield was not affected by plant density or by shading. Lower plant density was compensated by an increase in seed number per plant, without effects on single-seed weight. However, the negative effect of shading during flowering on seed number per area was fully compensated by an increase in single-seed weight by 47–61%. The plasticity of single-seed weight observed in the present study of oilseed rape has never been reported for annual seed crops. Shading at flowering increased both the seed filling rate and the duration of the seed filling period at all positions in the canopy. We also observed that the reduction of the source-sink ratio at flowering increasing seed weight does not necessarily modify oil or protein concentrations. Thus, single-seed weight could be targeted to increase seed yield in oilseed rape without compromising oil content. Understanding the response of the number of seeds and seed weight to the availability of assimilates is crucial for designing breeding strategies aimed to increase seed and oil yield in oilseed rape. This study aims to answer the questions: i) do seed number and seed weight in oilseed rape differ in their plasticity in response to the availability of assimilates at flowering? and ii) how sensitive are oil and protein concentrations to the availability of assimilates during flowering? A spring oilseed rape hybrid was sown in two field experiments and the treatments were combinations of i) two plant densities and ii) shading or no shading between the beginning and end of flowering. Seed yield was not affected by plant density or by shading. Lower plant density was compensated by an increase in seed number per plant, without effects on single-seed weight. However, the negative effect of shading during flowering on seed number per area was fully compensated by an increase in single-seed weight by 47–61%. The plasticity of single-seed weight observed in the present study of oilseed rape has never been reported for annual seed crops. Shading at flowering increased both the seed filling rate and the duration of the seed filling period at all positions in the canopy. We also observed that the reduction of the source-sink ratio at flowering increasing seed weight does not necessarily modify oil or protein concentrations. Thus, single-seed weight could be targeted to increase seed yield in oilseed rape without compromising oil content.