This book constitutes the refereed proceedings of the 11th International Conference on Open Semantic Technologies for Intelligent Systems, OSTIS 2021, held in Minsk, Belarus, during September 16-18, 2021.The 20 full papers included in this book were carefully reviewed and selected from 46 submissions. The papers are focused on the development of flexible and compatible technologies that provide fast and high-quality construction of intelligent systems for various purposes.