Atom probe tomography was used to achieve three-dimensional characterization of in situ Al2O3/GaN structures grown by metal organic chemical vapor deposition (MOCVD). Al2O3 dielectrics grown at three different temperatures of 700, 900, and 1000 °C were analyzed and compared. A low temperature GaN cap layer grown atop Al2O3 enabled a high success rate in the atom probe experiments. The Al2O3/GaN interfaces were found to be intermixed with Ga, N, and O over the distance of a few nm. Impurity measurements data showed that the 1000 °C sample contains higher amounts of C (4 × 1019/cm3) and lower amounts of H (7 × 1019/cm3), whereas the 700 °C sample exhibits lower C impurities (<1017/cm3) and higher H incorporation (2.2 × 1020/cm3). On comparing with Al2O3 grown by atomic layer deposition (ALD), it was found that the MOCVD Al2O3/GaN interface is comparatively abrupt. Scanning transmission electron microscopy data showed that the 900 °C and 1000 °C MOCVD films exhibit polycrystalline nature, while the ALD films were found to be amorphous.