A new analysis has been performed of Atmospheric Explorer C observations of O++ in the daytime thermosphere. The revised chemical model includes the rapid losses of O++ via N2 as reported from laboratory measurements and its production from direct double photoionization of O as an additional source. An improved procedure has been introduced for the analysis of the basic ion currents recorded by the magnetic ion-mass spectrometer and has special application for the very low concentrations near and below 200 km. Large-scale features in the low-altitude data are best represented with an additive term proportional to the N2 (or O2) density, with the preferred as a contaminant signal induced by neutral particles impinging on the instrument or spacecraft surfaces. This low altitude effect also accounts for otherwise excessive observed amounts of O++ previously reported near 200 km. The satellite data between 200 and 400 km are consistent with a rate coefficient of 6.6¿10-11 cm3/s for charge-exchange losses of O++ to O and with an observational estimate of 2.6¿10-9 s-1 for the production O++ from direct double photoionization for conditions near solar minimum. The latter ionospheric result also provides a reference value for the associated basic atomic photoionizaton cross section.