Cracking is observed when a UO_{2} single crystal is oxidised in air. Previous studies led to the hypothesis that cracking occurs once a critical depth of U_{3}O_{7} oxidised layer is reached. We present some \mu-Laue X-ray diffraction results, which evidence that the U_{3}O_{7} layer, grown by topotaxy on UO_{2}, is made of domains with different crystalline orientations. This observation was used to perform a modelling of oxidation coupling chemical and mechanical parameters, which showed that the domain patterning induces stress localisation. This result is discussed in comparison with stress localisation observed in thin layer deposited on a substrate and used to propose an interpretation of UO_{2} oxidation and cracking.