Decades of effort to develop precious-metal-free catalysts yielded few plausible candidates until the recent proliferation of Sb-based oxides, particularly in rutile structures containing first-row transition metals such as Ni, Mn, and Co. In addition to characterizing such catalysts, Mahidashti et al. provide an excellent summary of recent progress. Briefly, high throughput experiments from our group revealed that incorporation of Sb, Mn, and Co in an oxide catalyst (Mn-Co-Sb-O) provides an unprecedented combination of activity and stability. A similar combination of elements, Ni-Mn-Sb-O, in a rutile structure was identified, followed by our demonstration of further improving activity via transition metal enrichment within the rutile structure (Mn-Sb-O). Computational Pourbaix analysis revealed the thermodynamic basis for the stability of Mn-Sb oxides as well as the broader family of Sb-based rutile oxides under acid OER conditions. The family of catalysts was then expanded to include Co-Sb-O, prompting efforts to optimize properties via nanoparticle synthesis of M-Sb-O catalysts with M = Co, Mn, Ni, Fe, and Ru. Collectively, these efforts have demonstrated the stabilization of first row transition metals in Sb-based rutile structures, with a persistent challenge that increasing the Sb concentration increases stability while decreasing OER activity. Despite this challenge, the efforts to develop this family of catalysts continue because they provide the highest activity among nonprecious-metal acid OER electrocatalysts with dissolved metals concentrations below 10 -6 M, a measurement of stability that may serve as a proxy for long-term durability. In an effort to improve the activity of Sb-rich rutile oxides, Luke et al. embarked a combinatorial discovery effort based on M-Mn-Sb-O catalysts with M = Ru, Co, Pb, and Cr. Perhaps the most important observation from this work is the improved activity and stability of Mn-Pb-Sb-O catalysts, suggesting that rather than a diversity of transition metals, catalyst optimization may be realized with a diversity of other elements that stabilize Mn active sites. We recently used high throughput methods to identify compositionally complex rutile oxides (Co-Sn-Ta-O) where composition within the rutile phase tunes both OER activity and electrochemical stability. These collective experiences motivate the design of a combinatorial catalyst exploration in which Mn is combined with a variety of metals whose oxides are stable in acid OER conditions and are commonly found in rutile structures. Our original high throughput discovery efforts in the Mn-Co-Sb-Ta-O system may be considered the initial implementation of this strategy, although Ta did not elicit improved stability or activity. Given the excellent stability of rutile SnO2 and TiO2, we chose the following set of composition libraries to explore complex rutile oxides: Mn-Sn-Sb-O, Mn-Ti-Sb-O, and Mn-Sn-Ti-Sb-O. The Mn-Sb-O from our previous study 9 is a common subsystem that we measured as a baseline alongside the quaternary and quinary oxide systems. We designed combinatorial experiments to evaluate the 2-part hypothesis: