This book constitutes the refereed proceedings of the 12th International IFIP WG 2.13 International Conference on Open Source Systems, OSS 2016, held in Gothenburg, Sweden, in May/June 2016. The 13 revised full papers presented were carefully reviewed and selected from 38 submissions. The papers cover a wide range of topics related to free, libre, and open source software, including: organizational aspects of communities; organizational adoption; participation of women; software maintenance and evolution; open standards and open data; collaboration; hybrid communities; code reviews; and certification.