Neutral particle oscillations have proven to be extremely valuable probes of fundamental physics. Kaon oscillations provided us with our first insight into CP-violation, fast Bs oscillations provided the first indication that the top quark is extremely heavy, B oscillations form the most fertile ground for the continued study of CP-violation, and neutrino oscillations suggest the existence of a new, important energy scale well below the GUT scale. Neutrons oscillating into antineutrons could offer a unique probe of baryon number violation. The construction of the European Spallation Source in Lund, together with modern neutron optical techniques, offers an opportunity to conduct an experiment with three orders of magnitude improvement in sensitivity to the neutron oscillation probability.
