  Using observations from MAVEN's Neutral Gas and Ion Mass Spectrometer
(NGIMS), we characterize the seasonal, solar zenith angle (SZA), and solar flux
dependent variations of the O$^+$ peak and the O$^+$/O$_2^+$ ratio in the
topside ionosphere of Mars. We find that the O$^+$ peak is between 220-300 km
and forms at a roughly constant neutral atmospheric pressure level of
10$^{(-8.7 \pm 0.4)}$ Pa. The O$^+$ peak altitude also decreases with
increasing SZA near the terminator and varies sinusoidally with an amplitude of
26 km over a period of one Mars year in response to the changing solar
insolation. The O$^+$ peak altitude reaches a maximum near Northern Winter
solstice and Mars perihelion. The O$^+$ peak density on the dayside has an
average value of (1.1$\pm$ 0.5)$\times$ 10$^{3}$ cm$^{-3}$, has no dependence
on SZA for SZAs up to $\sim$90$^{\circ}$, and is mainly controlled by the
thermospheric O/CO$_2$ ratio as predicted by photochemical theory. Above the
O$^+$ peak, the O$^+$/O$_2^+$ ratio in the dayside ionosphere approaches a
constant value of 1.1 $\pm$ 0.6, decreases with increasing SZA, and is highly
variable on timescales of days or less. We discuss why the O$^+$ peak is
different than the main (M2) peak at Mars and why it is similar to the F2 peak
at Earth.
