The Mesospheric Semi-Annual Oscillation in the mean zonal winds (MSAO) is a prominent feature of the tropical middle atmosphere. This oscillation is thought to be driven by the selective transmission of small-scale gravity waves through the wind system of the stratospheric Semi-Annual Oscillation (SSAO). However, due to limited observations, the driving mechanisms of this oscillation are not completely understood. Here we examine the driving of the MSAO using the Whole Atmosphere Community Climate Model, version 2 (WACCM2). We show that near the solstices, in addition to gravity waves, meridional advection and EP flux divergence from resolved waves (in particular from the quasi two-day wave) contribute significantly to the momentum budget of the MSAO. Near the equinoxes, all forcing terms are weak suggesting that the easterly phase of the MSAO is driven by a process not well represented in WACCM2.