Solar wind interacts with atmospheres of planets, moons and comets. One of the important interaction processes is charge-exchange. High charge-state heavy ions in the solar wind are prone to charge-exchange with typical atmospheric constituents due to their large cross sections. These processes are important for remote observations of comets through X-ray emissions. We survey the ESA/Rosetta data for in-situ observations of multiply-charged oxygen ions and estimate the neutral outgassing rate of comet 67P/Churyumov-Gerasimenko through the fluxes of heavy ions such as O6+ and O5+. Methods. We use data from the Ion Composition Analyzer on board the Rosetta spacecraft to obtain oxygen ion fluxes. The expected signal is weak and therefore we apply careful data filtering. We then adapt a pre-existing model to calculate the outgassing rate of the comet. We find that multiply-charged oxygen ions are mostly found post perihelion. The estimated outgassing rate is QthO=1.87x10^28^Rh^-5.00^[s^-1^] with Rh the heliocentric distance. The outgassing estimates agree well with estimates from previous studies based on observations of helium ions. We extend these previous studies with a completely independent data set, with coverage until the end of the mission and by taking the varying neutral gas composition into account. The ratio of oxygen-to-helium fluxes also matches previous measurements. Our comparison to ROSINA neutral gas measurements shows that heavy solar wind ions, such as oxygen, can be used to retrieve the neutral gas activity of the comet, when direct neutral gas observations are not available.