If you watch frame by frame as the CO2 fuses, the temp and pressure in the reaction chamber should be sufficient to immediately fuse the OXYG.
Another thing I noticed is that pressure of a particular OXYG particle gets reset to 250.2 for some reason, even though other areas in the chamber are well above that pressure. Under normal conditions every pixel in that chamber registers identical pressure and temp.
Along a similar vein here is NBLE not fusing at the appropriate temp and pressure as stated on the wiki.