This book gathers a selection of refereed papers presented at the "International Conference on Operations Research OR2015," which was held at the University of Vienna, Austria, September 1-4, 2015. Over 900 scientists and students from 50 countries attended this conference and presented more than 600 papers in parallel topic streams as well as special award sessions. Though the guiding theme of the conference was "Optimal Decision and Big Data," this volume also includes papers addressing practically all aspects of modern Operations Research.