The origin of CO and the external source of H2O in the atmosphere of Titan is still a matter of debate. We investigated the chemical fractionation of oxygen isotopologues in order to give new constraints on the origin of oxygen species. We created a new chemical scheme and we developed a 1-D time-dependent photochemical model to compute the mole fraction profiles of various oxygen isotopologues. We show that the photochemical fractionation of oxygen isotopologues is quite low. Observations of C18O and CO18O are compatible with both an external origin or an internal origin of CO considering that the various sources of oxygen have a cometary 16O/18O ratio (16O/18O ≈ 500). Improvement of the measurements of the 16O/18O ratio in both Enceladus' plumes and atmospheric CO2 could give a valuable constraint on the origin of oxygen in Titan's atmosphere.