Crop diversification at the plot level, through intercropping or companion planting, involves growing two or more plant species in proximity and is increasingly adopted as a sustainable agronomic practice. Beyond optimizing land use and soil fertility, it can reduce insect pest impact. Such protection may arise directly, via visual, physical, or chemical disruption, or indirectly, through host plant modifications induced by the companion plant. Oilseed rape (OSR, Brassica napus), a major European crop, is highly susceptible to a complex of insect pests. Studying the OSR–faba bean (FB, Vicia faba) companion planting system offers insights into how crop diversification reduces pest pressure and the ecological mechanisms involved. Two complementary field trials were conducted under open-field conditions with natural insect immigration. The first trial, carried out over two consecutive years, aimed to evaluate the direct effects of FB companion plants on the incidence of the complex of OSR insect pests. The second trial focused on indirect effects by assessing how companion planting influences the morphology and metabolism of OSR plants, and how these changes may affect pest–host interactions with Psylliodes chrysocephala. FB companion plants provided physical and/or visual disruption, leading to a reduction in pest incidence across the OSR pest complex. However, this effect did not fully account for the overall decrease in pest pressure observed. Companion planting also induced significant morphological and metabolic modifications in OSR, altering ecophysiological traits such as morphological structure and metabolite profiles (glucosinolates). These changes likely influenced insect host selection, contributing further to pest suppression. The OSR–FB companion planting system reduces pest pressure through combined direct and indirect mechanisms, with physical and visual disruption playing a central role. These findings deepen understanding of how crop diversification disrupts insect pests and provide a framework to evaluate the mechanisms underlying pest regulation in diversified cropping systems.