Abstract: In complex socio-technical systems, robustness is achieved through interaction between the technical structure of the system and the social and organizational structure of the operators who run the system. While the need for human oversight of complex systems is widely recognized, the impact of organizational factors on the effectiveness of the oversight function is not well understood. We have studied the social interactions between supervision and maintenance operators of the largest French telecom operator, using techniques from the sociology of organizations. Detailed analysis of the social network formed by these operators has allowed us to identify a number of factors that contribute positively or negatively to the robustness of the system.