Novel catalytic architectures composed of catalytic centers with sub-nanometer diameters for CO oxidation reaction were designed, synthesized, and characterized. Accordingly, well-dispersed iridium precious metal active sites were supported on various catalytic support materials. Namely, magnesium oxide (MgO), ceria (CeO2), lanthana-zirconia (La2O3–ZrO2) and titania-zirconia (TiO2–ZrO2) systems were chosen as different support systems. The favorable catalytic effect of highly-dispersed Ir active sites with sub-nanometer diameters were demonstrated in flow-mode catalytic performance tests, where the lower loadings of highly dispersed Ir sites showed comparable catalytic activity in CO oxidation to that of bigger Ir clusters with higher metal loading. Furthermore, influence of the catalyst pre-treatment conditions (e.g., reduction in H2, oxidation in O2, and calcination in air) on the catalyst structure and performance were also studied via XRD, Raman, BET, XPS, TEM, EDX, and in-situ FTIR spectroscopy techniques. Our results indicate that in all the catalytic systems, high-dispersion Ir sites can be generated on supports where Ir exists as small clusters with < 1 nm particle size. Moreover, catalyst pretreatment conditions revealed noticeable alterations in the catalyst structure in terms of average support particle size, reduction extent of the support, specific surface area, pore volume, pore size, and Ir oxidation state. Finally, catalytic performance results indicated that under reaction conditions yielding close to 100% CO conversion, 0.2 and 0.5 wt.% Ir catalysts led to comparable performance to that of 1 wt.% Ir catalyst demonstrating the advantage of catalytic systems with highly dispersed sub-nanometer diameter active sites with extremely low metal loading.