ESSO is an English language training course for NATO and PfP staff officers. The purpose of conversion of the original paper/audio cassette format into an on-line course is to address the needs of officers participating in multinational missions in operational English and to facilitate the language learning process by the use of modern technologies. The paper discusses course planning and design and presents features available in the on-line version, such as automatic feedback to the student, links to related sites and supplementary materials, animation, and electronic dictionaries. Pilot testing confirmed usefulness of the course, but at the same time brought some students’ concerns about potential technical difficulties and availability of instructor’s support.