The powerful x-class flare which occurred on the Sun on 28 October 2003 had important effects on plasma environments throughout the solar system. We present here observations of the effects at Mars from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer experiment. In particular we focus on the changes in the nature of the magnetic oscillations observed at an altitude of 400 km (MGS's current orbital altitude) during the passage of the solar storm. We find that strong, regular oscillations are observed in both the B$parallel$ and B$perp$ components of the magnetic field at all solar zenith angles. We emphasize in particular the powerful, coherent oscillations observed in the normally quiet nightside region. These oscillations carry power at the proton gyrofrequency and at and below the oxygen gyrofrequency. This implies that ions of planetary origin are interacting with the solar wind plasma and raises the possibility that significant atmospheric loss may occur during the passage of large solar storms at Mars.