A nanosecond repetitively pulsed discharge in a quartz capillary filled with flowing synthetic air was investigated as a benchmark to address the mechanism of fast gas heating for conditions of up to complete dissociation of O2 and heating of a few thousand K occurring during the near afterglow phase. Gas temperature, energy deposition, and O atom concentrations were measured with respect to time. O atom results are compared to the output from a detailed 0D kinetic model describing the excited species chemistry. The high oxygen dissociation degree enables investigation of the key role played by O atoms in fast gas heating of nanosecond discharge plasmas at high specific energy loading.