The effectiveness of remedial mathematics courses in post-secondary education has been a controversial topic for years. Higher Education institutions need their students to have basic understandings of the subjects to be imparted in the first semesters, but since they come with different backgrounds and prior knowledge, this is not always possible and many students struggle in their first courses. This paper presents the results of students' adoption and learning outcomes of using four MOOCs as a complementary study resource for an on-campus calculus diagnostic exam. Over 700 newly admitted university students had to take a mandatory diagnostic exam on four calculus topics before classes started. MOOCs were proposed as a voluntary support for studying these subjects. Following a mixed method analysis, we studied why and when the students used the online courses and we also measured the effects of its use in terms of the students' diagnostic exam grades and learning outcomes. The results show that students mostly used the MOOCs to study the subjects that were not covered in their secondary studies. Students who were active in these course topics obtained better scores, having more chances of passing the diagnostic exam than students who did not study with the MOOCs. Furthermore, students not only used the MOOCs for studying for the exam, but also for refreshing concepts for future courses.