Operating Systems is a difficult subject to learn; however, little is known about these difficulties, as they have not been studied or determined by the relevant literature. The objective of this article is to specify the most difficult concepts for understanding the subject and misconceptions that students have regarding these concepts. The study was conducted through an online university over the course of an entire semester. The research data comes from the evaluations taken by nine students, which have been analyzed using qualitative methods. The most difficult concepts for students to understand include concurrent computing and the mechanisms to change the program that the processor is running (interrupts, context switches, system calls, etc.). Six misconceptions regarding these concepts have been identified, helping to determine the specific problems that need to be resolved.