Introduction: Functional traits, such as morphology, phenology, and chemical defenses, are key factors of plant–plant and plant–environment interactions and are central to predicting ecosystem responses in managed agrosystems. Growing oilseed rape (OSR, Brassica napus) with legumes, as a key agroecological lever, can enhance yield and sustainability. However, the trait-based mechanisms underlying these benefits remain unclear. Methods: We investigated the morphological development, phenology, and glucosinolate defense traits of three OSR varieties grown as monocrops or grown with faba beans (FB, Vicia faba) companion plants under controlled glasshouse and two-year field conditions to explore competition and/or facilitation processes. We further characterized the cropmicroclimate (temperature, light quantity, light quality) and monitored insect pests to link OSR trait responses with agronomic outcomes. Results: Growing OSR with FB companion plants consistently reduced OSR leaf development both in the greenhouse and in the field due to reduced photosynthetically active radiation under the FB canopy, revealing light-driven plasticity in early vegetative traits. At the reproductive stage, trait expression diverged across environments: in glasshouse conditions, excessive FB competition suppressed OSR reproductive development, while in the field, winter froze FB stems, which alleviates shading, triggering compensatory stem elongation, increased branching, and enhanced yield of OSR. Yield gains were thus mediated by morphological and phenological trait adjustments rather than direct pest suppression. Glucosinolate concentrations and profiles were strongly shaped by environment and variety, but companion planting notably decreased OSR total glucosinolates after winter in the field, indicating context-dependent defense trait responses. Discussion: Our findings highlight how environmental context and varietal identity govern the expression of aboveground functional traits in OSR–FB companion planting systems. Beyond its contribution to pest regulation and despite contrasting effects on plant traits throughout development, OSR–FB companion planting demonstrated agronomic viability by promoting OSR adaptive responses that maintained productivity under field conditions.