Without newcomers, open source software (OSS) projects are hardly sustainable. Yet, newcomers face a steep learning curve during OSS onboarding in which they must overcome a multitude of technical, social, and knowledge barriers. To ease the onboarding process, OSS communities are utilizing mentoring, task recommendation (e.g., “good first issues”), and engagement programs (e.g., Google Summer of Code). However, newcomers must first cultivate their motivation for OSS contribution and learn the necessary preliminaries before they can take advantage of these mechanisms. We believe this gap can be filled by a dedicated, practice-oriented OSS onboarding course. In this paper, we present our experience of teaching an OSS onboarding course at a top university in Asia. The course contains a series of lectures, labs, and invited talks to prepare students with the required skills and motivate them to contribute to OSS. In addition, students are required to complete a semester-long course project in which they plan and make actual contributions to OSS projects. They can either 1) contribute to one of the given OSS projects with dedicated mentoring from the course, or 2) contribute to any OSS project they prefer without such mentoring. Finally, 16 out of 19 students have successfully contributed to open source and five retained. However, the onboarding trajectories and outcomes differ vastly between the two groups of students with different course project choices, yielding lessons for software engineering education.