This book constitutes the refereed proceedings of the 5th IFIP WG 2.13 International Conference on Open Source Systems, OSS 2009, held in Skovde, Sweden, in June 2009. The 28 revised full papers presented together with the abstracts of two keynote speeches, 6 workshop introductions, 5 posters and 2 panels were carefully reviewed and selected from 59 submissions. The papers reflect the international communities of active OSS researchers from a number of academic fields ranging from computer science and information science to economics and business studies. They feature topics extending from architectures of OSS, mining OSS data, empirical research on OSS, user involvement in OSS design and OSS communities to commercial OSS, company participation in OSS, and public sector and educational usage of OSS.