Fairness emerges as an important research issue in overlay multicast because spreading the multicasting load evenly among participants can eliminate potential traffic hot spots, thus improving the system?s Quality of Service (QoS). FairOM [1, 2] has been proposed to enforce participants to contribute the same proportion of their available outgoing bandwidth to each session. With FairOM, more multicast sessions can be enabled simultaneously that would otherwise be impossible. In this paper, we analyze FairOM and compare it with non-FairOM approaches from two aspects: tree height and number of sessions that can be supported, which measure FairOM from a single-session?s and multiple-session?s point of view, respectively. Together, they draw an overall picture of FairOM. In this analysis, we make the following assumptions.