The Far Ultra Violet (FUV) ultraviolet imager onboard the NASA-ICON mission is dedicated to the observation and study of the ionosphere dynamics at mid and low latitudes. We compare O+ density profiles provided by the ICON FUV instrument during nighttime with electron density profiles measured by the COSMIC-2 constellation (C2) and ground-based ionosondes. Co-located simultaneous observations are compared, covering the period from November 2019 to July 2020, which produces several thousands of coincidences. Manual scaling of ionogram sequences ensures the reliability of the ionosonde profiles, while C2 data are carefully selected using an automatic quality control algorithm. Photoelectron contribution coming from the magnetically conjugated hemisphere is clearly visible in FUV data around solstices and has been filtered out from our analysis. We find that the FUV observations are consistent with the C2 and ionosonde measurements, with an average positive bias lower than 1 × 1011 e/m3. When restricting the analysis to cases having an NmF2 value larger than 5 × 1011 e/m3, FUV provides the peak electron density with a mean difference with C2 of 10%. The peak altitude, also determined from FUV observations, is found to be 15 km above that obtained from C2, and 38 km above the ionosonde value on average.