Ba deposition on a theta-Al2O3/NiAl(100) substrate and its oxidation with gas-phase O-2 at various surface temperatures are investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD) techniques. Oxidation of metallic Ba by gas-phase O-2 at 800 K results in the growth of 2D and 3D BaO surface domains. Saturation of a metallic Ba layer deposited on theta-Al2O3/NiAl(100) with O-2(g) at 300 K reveals the formation of BaO2-like surface states. These metastable peroxide (O-2(2-)) states are converted to regular oxide (O2-) states at higher temperatures (800 K). In terms of thermal stability, BaO surface layers (theta(Ba) < 5 ML) that are formed by O-2(g) assisted oxidation on the theta-Al2O3/NiAl(100) substrate are significantly more stable (with a desorption/decomposition temperature of c. a. 1050 K) than the thick (2 < theta(Ba) < 10 ML) metallic/partially oxidized Ba layers prepared in the absence of gas-phase O-2, whose multilayer desorption features appear as low as 700 K.