Context: Open Source Software (OSS) is a vital public good, included across most of modern software stacks, significantly impacting GDP and national tech growth, while supporting interoperability, sovereignty, and transparency. However, systematic measurement of governmental OSS adoption remain limited. Research Aim: This study contributes to digital government maturity indexes by analyzing policies and support actions leveraging OSS for software reuse and collaborative development across 16 digitally mature countries, and proposing potential indicators for said indexes. It examines OSS policy formation, stated goals, key actors, and support mechanisms. Methodology: A qualitative approach is used combining desk research of policy documents with semi-structured interviews of government representatives, producing detailed country reports. These are cross-analyzed, focusing on OSS policy promotion, rationale, and implementation support. Results: Policies facilitating OSS reuse are widespread, targeting both inbound acquisition and outbound sharing, and are predominantly governed by central public sector organizations. Policy goals include interoperability, digital sovereignty, transparency, and cost efficiency, with security framed both as a risk and strength. Implementation is supported by diverse Open Source Program Offices (OSPOs) at multiple government levels, which foster capacity building, resource pooling, and sustainable project governance. Indicators are synthesized and proposed across 14 areas covering policy incentives and design, and implementation and support. Conclusions: OSS is a strategic enabler for public sector digital transformation. Clear policy frameworks, coupled with institutional support such as OSPOs, are essential. International digital maturity frameworks should expand OSS indicators to better guide and assess government adoption and impact.