Winter oilseed rape (OSR) can be grown intercropped with frost-sensitive service plant mixtures. This practice may reduce weed pressure and contribute to providing N for OSR after service plant freezing. However, there is little knowledge of how plants interact together and with the soil in diverse annual crop mixtures. To assess these interactions, two contrasting legume service plants were selected: faba bean and grass pea, as well as a non-legume, the niger plant. OSR and the legumes were then grown in mesocosms alone or intercropped in mixtures of two to four species. The biomass, N contents, N2 fixation, and soil substrate-induced respiration were measured. A single species mostly drove the total dry biomass and the amount of N accumulated by the cover, OSR and faba bean for dry weight and N contents, respectively. Grass pea was highly sensitive to competition with OSR, and its N2 fixation was lower than that of faba bean. The addition of niger did not lead to additional N2 fixation of legumes or total N accumulation and contributed to reducing OSR biomass. The specific composition of the plant mixture did not explain the soil microorganism activity.