This book constitutes the refereed proceedings of the 11th International IFIP WG 2.13 International Conference on Open Source Systems, OSS 2015, held in Florence, Italy, in May 2015. The 16 revised full papers presented were carefully reviewed and selected from 50 submissions. They have been organized in the following topical sections: open source software engineering; communication and collaboration; examples and case studies; adoption, use, and impact; and intellectual property and legal issues. Also included is an OSS 2015 PhD contest paper.