We report Magnetospheric Multiscale observations of large amplitude, parallel, electrostatic, proton plasma frequency waves on the magnetospheric side of the reconnecting magnetopause. The waves are often found in the magnetospheric separatrix region and in the outflow close to the magnetospheric ion edge. Statistical results from five months of data show that these waves are closely tied to the presence of cold (typically tens of eV) ions, found for 91% of waves in the separatrix region, and that plasma properties are consistent with ion acoustic wavegrowth. We analyze one wave event in detail, concluding that the wave is ion acoustic. We provide a simple explanation for the mechanisms leading to the development of the ion acoustic instability. These waves can be important for separatrix dynamics, by heating the cold ion component and providing a mechanism to damp the kinetic Alfvén waves propagating away from the reconnection site.