Even though many countries in Latin America have adopted FOI Laws, there are significant differences in the institutional design of FOI oversight institutions. Most explanations highlight the role of political competition in motivating political actors to design strong de jure FOI oversight institutions. The design of FOI oversight institutions in Chile, Peru and Uruguay, however, cannot fully be explained by political competition. We show how isomorphic pressures help explain variation in the de jure strength of the FOI oversight institutions. Our findings highlight the importance of considering domestic constraints on the diffusion of one-size-fits-all models. To analyze each case, we conducted a systematic process-tracing analysis. Our in-depth analysis allowed us to assess different theories concerning the specific institutional design of FOI oversight institutions.