A well-known way to help newcomers overcome initial contribution challenges is mentoring. This strategy has proven effective in offline and online communities, and to some extent has been employed in Open Source Software (OSS) projects. Through mentoring, newcomers are trained to acquire the technical, social, and organizational skills they need. Despite the importance of OSS mentors, they are under studied in the literature. Understanding who mentors in OSS projects are, the challenges they face, and the strategies they use can help OSS projects better understand and support mentors' work. In this paper, we investigate the OSS mentors' perspectives by employing a two-stage study. First, we understand the characteristics of the mentors in a large OSS community through a large-scale online survey in the Apache Software Foundation. We found that contributors who are volunteers and less experienced are less likely to take on the role of mentoring. Second, we identify the challenges that mentors face and how they mitigate these challenges through interviews with OSS mentors (n=18). In total, we identified 25 general mentorship challenges and 7 sub-categories of challenges regarding task recommendation. We also identified 13 strategies to overcome these challenges. Our results provide insights for OSS communities, formal mentorship programs like Outreachy, and tool builders who design automated support for task assignment and internship.