This book gathers a selection of peer-reviewed papers presented at the International Conference on Operations Research (OR 2021), which was hosted online by the University of Bern from August 31 to September 3, 2021, and was jointly organized by the Operations Research Societies of Switzerland (SVOR/ASRO), Germany (GOR e.V.), and Austria (OEGOR). The respective papers discuss classical mathematical optimization, statistics and simulation techniques. These are complemented by computer science methods, and by tools for processing data, designing and implementing information systems. The book also examines recent advances in information technology, which allow massive volumes of data to be processed and enable real-time predictive and prescriptive business analytics to drive decisions and actions. Lastly, it presents a selection of problems that are modeled and treated while taking into account uncertainty, risk management, behavioral issues, etc.