The impact of catalyst pretreatment and its role in diesel oxidation catalysts was systematically studied by comparing the performance, CO adsorption characteristics, oxidation state, and local structure of Pt for Pt/CeO2-Al2O3 and Pt/Al2O3. The catalysts were characterized during oxidizing and reducing conditioning (conventional/reducing pulses) and the subsequent CO oxidation light-offs by operando X-ray absorption spectroscopy (XAS) and insitu diffuse reflectance infrared Fourier transform spectroscopy. The results indicate that a reducing pretreatment substantially enhances the CO oxidation activity of the ceria-based catalyst (70 degrees C lower light-off temperature). Operando XAS uncovered the highly dynamic nature of the Pt state in both catalysts. The oxidation state and particle size gradually change, and high CO oxidation activity was only observed after a partial reduction of the noble metal. Particularly, the oxidizing conditioning step resulted in a different noble metal response depending on the carrier material. Furthermore, pronounced differences were identified by correlating the Pt structure and catalytic activity: While for Pt/Al2O3, high activity was not observed when Pt was still covered by CO, high CO conversion (>80%) was encountered for Pt/CeO2-Al2O3 already at low temperatures, demonstrating the absence of a CO inhibition effect. The data underline the prominent role of the Pt-CeO2 interface to circumvent Pt self-poisoning by CO.