We present a combined low energy electron microscopy and photoemission electron microscopy study of Co nanoislands grown on pure and partially oxidized Ru(0001) surfaces at 650--850 K. When Co is deposited on pure Ru(0001) at the temperature above $\ensuremath{\approx}700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it first forms a wetting monolayer and then Co nanoislands abruptly nucleate. Partial oxidization of the clean Ru(0001) surface gives rise to large rhombus-shaped Ru--O patches (of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ scale). On this surface at $\ensuremath{\approx}730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Co islands form with different density in the Ru and Ru--O regions, and at $\ensuremath{\approx}800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Co islands only grow in the pure Ru regions. We attribute this selectivity to differences in the temperature dependence of the sticking coefficients for Co on the pure Ru and Ru--O regions.