We report on the first-principles-guided design, synthesis, and characterization of core shell nanoparticle (NP) catalysts made of a transition metal core (M = Ru, Rh, Ir, Pd, or Au) covered with a similar to 1-2 monolayer thick shell of Pt atoms (i.e., a M@Pt core shell NP). An array of experimental techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and temperature-programmed reaction, are employed to establish the composition of the synthesized NPs. Subsequent studies of these NPs' catalytic properties for preferential CO oxidation in hydrogen-rich environments (PROX), combined with Density Functional Theory (DFT)-based mechanistic studies, elucidate important trends and provide fundamental understanding of the reactivity of Pt shells as a function of the core metal. Both the PROX activity and selectivity of several of these M@Pt core shell NPs are significantly improved compared to monometallic and bulk nonsegregated bimetallic nanoalloys. Among the systems studied, Ru@Pt core shell NPs exhibit the highest PROX activity, where the CO oxidation is complete by 30 degrees C (1000 ppm CO in H-2). Therefore, despite their reduced Pt content, M@Pt core shell NPs afford the design of more active PROX catalysts. DFT studies suggest that the relative differences in the catalytic activities for the various core shell NPs originate from a combination of (i) the relative availability of CO-free Pt surface sites on the M@Pt NPs, which are necessary for O-2 activation, and (ii) a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.