Open source is an important model of collaborative knowledge work and virtual organizations. One of its work practices, peer review, is considered critical to its success, as Linus's law highlights. Thus, understanding open source peer review, particular effective review practices, will improve the understanding of how to support collaborative work in new ways. Therefore, we conduct case studies in two open source communities that are well recognized as effective and successful, Mozilla and Python. In this paper, we present the preliminary results of our analysis on data from the bug tracking systems of those two organizations. We identify four common activities critical to open source software peer review, submission, identification, resolution and evaluation. Differences between communities indicate factors, such as reporter expertise, product type and structure, and organization size, affect review activities. We also discuss features of open source software peer review distinct from traditional review, as well as reconsiderations of Linus's law.