We investigate the escape rates of O+ through the magnetotail of Venus and its dependence on the upstream solar and solar wind conditions, using Venus Express measurements. We find that the O+ escape rate increases with the solar wind energy flux as Q=Q_0*F(SWenergy)^(0.5+-0.3), where Q_0 = 7.1*10^16 for high EUV and Q_0 = 8.5*10^16 for low EUV. As the solar EUV flux did not increase significantly over the studied solar cycle, the variation of the escape rates with the solar EUV flux is not strong in this dataset. Nevertheless, the escape rate decreases with higher EUV as there is an increase in the Venusward fluxes. From the relation between the escape rate and the solar wind energy flux we extrapolated the escape rates to 3.9 Ga. The results indicate a total loss of 3.6*10^16 kg of water through non-thermal ion escape, or equal to ~0.3 m of a global equivalent layer of water, and therefore cannot account for the loss of an historical terrestrial-like ocean (~10^21 kg) in the Venusian atmosphere.