Nanoscopic clustering in a 2D disordered phase is observed for oxygen on Ru(0001) at low coverages and high temperatures. We study the coexistence of quasi-static clusters (with a characteristic length of ∼ 9Å) and highly mobile atomic oxygen which diffuses between the energy-inequivalent, threefold hollow sites of the substrate. We determine a surprisingly low activation energy for diffusion of 385 ± 20 meV. The minimum of the O − O interadsorbate potential appears to be at lower separations than previously reported.