After visual inspection, these sites are often found on dangling platinum atoms or on the edge of the nanoparticle. These sites were previously reported to be the most stable sites for atomic oxygen binding by DFT studies on smaller nanoparticles. Finally, as the pressure increases 4-fold subsurface sites become more populated, which occurs between 10 -10 and 10 -2 atm when the outer coverage is between 0.7 and 0.9 ML. As suggested by previous DFT studies subsurface sites require sufficient lattice disruption to be populated. In our case, sufficient deformation is reached due to oxygen adsorption at the outer shell. We confirmed this hypothesis by performing a full DFT calculation on a smaller nanoparticle with a single oxygen in a subsurface site. We find that the lone oxygen atom moved onto the surface at a 3-fold coordination site during the geometry optimization. Details including pictures of this auxiliary calculation can be found in the supplementary information.