This open access book constitutes the refereed proceedings of the 15th IFIP WG 2.13 International Conference on Open Source Systems, OSS 2019, held in Montreal, Quebec, Canada, in May 2019. The 10 revised full papers and 5 short papers presented were carefully reviewed and selected from 35 submissions. The papers cover a wide range of topics in the field of free/libre open source software (FLOSS) and are organized in the following thematic sections: mining OSS data; organizational aspects of FLOSS projects; FLOSS adoption; FLOSS cost and licenses; and FLOSS education and training.