[en] A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WDs) in a binary. The observed ratio of ¹⁶O/¹⁸O for RCB stars is in the range of 0.3-20 much smaller than the solar value of ∼500. In this paper, we investigate whether such a low ratio can be obtained in simulations of the merger of a CO and a He WD. We present the results of five three-dimensional hydrodynamic simulations of the merger of a double WD system where the total mass is 0.9 M_☉ and the initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations with q ∼< 0.7 a feature around the merged stars where the temperatures and densities are suitable for forming ¹⁸O. However, more ¹⁶O is being dredged up from the C- and O-rich accretor during the merger than the amount of ¹⁸O that is produced. Therefore, on the dynamical timescale over which our hydrodynamics simulation runs, an ¹⁶O/¹⁸O ratio of ∼2000 in the 'best' case is found. If the conditions found in the hydrodynamic simulations persist for 10⁶ s the oxygen ratio drops to 16 in one case studied, while in a hundred years it drops to ∼4 in another case studied, consistent with the observed values in RCB stars. Therefore, the merger of two WDs remains a strong candidate for the formation of these enigmatic stars.