Little is known about the experiences of the social partners in helping undeclared workers resist Occupational Safety and Health (OSH) issues. This study draws upon Walter Korpi's ‘power resource theory’ to gain a deeper understanding of how power resources within the construction, transport, and cleaning sectors influence the ability of social partners to respond to OSH issues related to undeclared work. This mixed-method study uses survey data from employer representatives in the construction (n = 686) and transport (n = 650) sectors in Sweden in 2019 to estimate the nature and magnitude of undeclared work-related problems. To also study the view of union representatives, a duplicate survey was sent to union representatives in the transport, construction, and cleaning sectors (n = 57) in 2020, followed by 13 semi-structured interviews with Regional Safety Representatives (RSRs) in 2021–2023. Our findings show that employer representatives in construction and transport reported that the violation of OSH regulations was uncommon and remained unchanged, most union representatives said the opposite. We found a gradient of activism among the unions towards OSH issues related to undeclared work dependent on their power resources. Furthermore, structural and organizational factors limited the RSRs’ ability to address undeclared work. The RSRs identified strategies to tackle OSH issues related to undeclared work in their sectors, these included but were not limited to, dismantling the language barrier between unions and undeclared foreign-born workers, for OSH coordinators and main contractors to be held responsible for OSH violations and greater cooperation between the relevant authorities dealing with undeclared work.