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The energy resolution of an indirect time of flight (tof) spectrometer is determined mainly by the pulse shape of the incoming pulse and the contribution of the crystal analyser. We performed extensive Monte Carlo simulations for the indirect near-backscattering spectrometer OSIRIS utilising the McStas neutron ray-traycing package. The simulations are accompanied by analytical calculations for the energy resolution. From simulation and calculation an excellent description for the width of the line is achieved for the PG002 and PG004 energy setting. The simulations and calculations reveal that the secondary spectrometer and hence the analyser geometry is the dominating term for the energy resolution at zero energy transfer. The remaining differences in the lineshape can be traced to a not perfectly modeled hydrogen moderator. The simulations and calculations predict a superb energy resolution of less than 100 μeV at an energy transfer of 15 meV. [ABSTRACT FROM AUTHOR]