During storm-time substorms the IMAGE/High-Energy Neutral Atom (HENA) global magnetospheric plasma imager observes dramatic rise and fall of the oxygen energetic neutral atom (ENA) fluxes in the 29--180 keV range that are well correlated with substorm onsets. The corresponding hydrogen ENA flux (10--198 keV) shows a much more gradual rise and fall. We use a three-dimensional test particle code in time varying electric and magnetic fields to study mechanisms causing the dramatic oxygen variation. Consistent with our previous work, we find that nonadiabatic heating occurs during the dipolarization of the geomagnetic field. The simulation model reproduces key features including the high-altitude increase of the ENA flux and enhancement timescale.