In the quest for an electrical insulation gas with low global warming potential for medium and high voltage equipment, perfluoronitrile (C4F7N) and perfluoroketone (C5F10O) based gas mixtures have shown a high electric strength, low toxicity and lower GWP than that of SF6, making them promising alternatives. Detailed swarm parameter investigations have already been made in binary mixtures with N2 and CO2. Manufacturers started to use mixtures of C4F7N or C5F10O with CO2 and O2, or with air, as insulating and switching media in medium and high voltage gas insulated substations. The addition of oxygen has shown to reduce the creation of soot and CO from discharges or sparks. The influence of the addition of oxygen on the electrical properties, in turn, has not yet been studied in detail and is the topic of the present publication. The electrical insulation properties of these gas mixtures are studied using a pulsed Townsend experiment. The effective ionization rate coefficient, the electron drift velocity and the longitudinal diffusion coefficient are obtained, and the density-reduced critical electric field is deduced for different mixtures containing up to 19% O2. For C4F7N:CO2:O2 and C5F10O:CO2:O2 mixtures in which the fluorinated gas is kept at 5%, the addition up to 10% O2 does not change the effective ionization rate coefficient. On the other hand, a clear increase in the latter is observed for mixtures of C5F10O:N2:O2 containing up to 19% O2 therefore having a negative impact on the insulation strength.