Courses on Operating Systems (OS) are essential in computer science education. The topic provides the students with an excellent opportunity to experience the interplay between theory and practice. Specifically, a project-based-learning (PBL) instructional design for an OS course can provide the students with opportunities to engage in practical projects. The PBL approach enables students to take part in learning activities which are essential for grasping underlying theoretical concepts, linking theory, practice and system-level perception. In this paper we present our experience in teaching an operating systems course with a continuing evolving project using a PBL approach. The findings of a preliminary assessment indicated a highly positive attitude on the part of the students towards the PBL approach used in the course as well as towards the qualitative evaluation method that was used to assess their achievements.