High-temperature operations present a substantial challenge for developing industrial-capable precious metal catalysts (e.g. Pt/alumina), where the scarce metal atoms easily become wasted as sintered nanoparticles. Using fully dispersed precious metal atoms to deliver the catalysis remains an elusive goal. On a widely used La-stabilized alumina support, we find that the atomically dispersed Pt-1(II)-O-x- species, rather than the much more visible large metal particles, are the actual catalytic sites for the CO and C3H6 oxidation reactions. Unfortunately, the La dopants in alumina support will not hinder the sintering of the active Pt-1(II)-O-x- species. As a natural next step, the Ba-O-x- species were introduced to specifically stabilize the single-atom Pt on the La-stabilized alumina support. By implementing this improved formulation, the atomically dispersed Pt on alumina retains the original full dispersion even after 650 degrees C hydrothermal aging. Intriguingly, with or without the barium additives and/or sintered platinum particles in the catalysts, the intrinsic activity per Pt atom stays intact. Along with other experimental evidence, this leads to the finding that the single-atom Pt is the true catalytic site for the oxidation of CO and C3H6 in the widely-used Pt/La-Al2O3 material system. This work provides a new perspective for efficient precious metal utilization under demanding catalytic conditions.