Ionospheric topside O⁺ diffusive flux is derived using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation data, to investigate its global distribution and also its role in winter nighttime enhancement (WNE) of electron density. The flux of the winter hemisphere maintains downward throughout the night. It is much larger between 30^o and 50^o geomagnetic latitudes and keeps increasing until 22:00-00:00 LT. It peaks at 60^oW and 60^oE-120^oE geographic longitudes during the December solstice, and at 180^oE during the June solstice. These features are similar to those of WNE in NmF2. Furthermore, the derived flux is applied as the upper boundary condition to run the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). The simulated spatial-temporal variations of WNE are consistent with the observations. The results indicate that downward plasma diffusion from the plasmasphere is the major mechanism of WNE, and the simulation quantifies its contribution.