The CCI/C3S level-2 total ozone Essential Climate Variable product contains harmonized total ozone vertical column densities retrieved from a number of nadir space sensors. The present data set provides ozone total columns as well geophysical information for ground pixels observed by the Ozone Monitoring Instrument (OMI, aboard the EOS-Aura satellite; ascending node equator crossing time: circa 13:40 hrs local time) for the period October 2004 onwards. The product also contains diagnostic metrics such as the results of the spectral fit, averaging kernel, a priori profiles, cloud information, and algorithm and product uncertainty estimates.