The adsorption of graphene-oxide (GO) nanoparticles at the interface between water and vapor was analyzed using all-atom molecular simulations for single and multiple particles. For a single GO particle, our results indicate that the adsorption energy does not scale linearly with the surface coverage of oxygen groups, unlike typically assumed for Janus colloids. Our results also show that the surface activity of the particle depends on the number of surface oxygen groups as well as on their distribution: for a given number of oxygen groups, a GO particle with a patched surface was found to be more surface active than a particle with evenly distributed groups. Then, to understand what sets the thickness of GO layers at interfaces, the adsorption energy of a test GO particle was measured in the presence of multiple GO particles already adsorbed at the interface. Our results indicate that in the case of high degree of oxidation, particle–particle interactions at the water–vapor interface hinder the adsorption of the test particle. In the case of a low degree of oxidation, however, clustering and stacking of GO particles dominate the adsorption behavior, and particle–particle interactions favor the adsorption of the test particle. These results highlight the complexity of multiple particle adsorption and the limitations of single-particle adsorption models when applied to GO at a relatively high surface concentration.