We show in the ESI the XRD and OER activity characterization of a Mn-Co-Sb-O composition library (Fig. ). This library contains a smaller fraction of phase-pure rutile samples due to the formation of (Mn,Co)3O4 at Co and Mn-rich compositions. Fig. shows that, with respect to the Mn-Sb-O baseline system, Mn may be substituted by Co and retain appreciable OER activity. However, given that Co is similar to Mn in its propensity for corrosion under acid OER conditions, no compositions in this library substantially outperform the Mn-Sb-O baseline with respect to high OER activity at low concentration of elements prone to corrosion. This observation highlights the opportunities for future catalyst development that build upon our discovery of complex rutile alloys in the Mn-Sb-Sn-Ti-O system (Fig. ) and their high OER activity at low Mn concentration (Figs. ). In addition to detailed investigation of catalyst durability, we envision that future can address the mechanism of activity enhancement in the complex rutile oxide catalysts, which may relate to the distribution of surface sites introduced by compositionally complex catalysts or the introduction of near-surface oxygen vacancies due to heterogeneity in the oxidation state of the 3 or 4 cation elements in the rutile phase.