One of the most important decisions coalition partners make when forming a government is the division of ministries. Ministerial portfolios provide the party in charge with considerable informational and agenda-setting advantages, which parties can use to shape policies according to their preferences. Oversight mechanisms in parliaments play a central role in mitigating ministerial policy discretion, allowing coalition partners to control each other even though power has been delegated to individual ministers. However, we know relatively little about how such mechanisms influence the agenda-setting and gatekeeping powers of ministers and how much influence minister parties have on policy output relative to the government as a whole in different institutional settings. We fill this gap by analyzing original data on over 2000 important social and economic policy reform measures adopted in nine Western European countries over 20 years, based on a coding of more than 1200 country reports issued by the Economist Intelligence Unit and the Organisation for Economic Co-operation and Development (OECD). We find that parliaments with strong oversight powers constrain the agenda-setting capacity of minister parties but have limited impact on their gatekeeping capacity. Our findings have important implications for our understanding of policy-making and democratic accountability.