High entropy chemical compositions is a rapidly emerging approach to design functional oxides. Following early studies of mechanical properties of metal alloys with 5 or more elements in single phase solid solution, 1,2 high entropy has later been explored for oxides and ceramics, e.g. rock salts, 10 perovskites, oxyfluorides, and carbides. High entropy compounds are usually defined by having a configurational entropy, Sconfig, larger than 1.5R, where R is the universal gas constant, which is achieved by 5 or more equimolar elements on the same sublattice. High configurational entropy can stabilize single-phase materials even if the compositional end members are unstable. High ionic conductivity is a prominent example of enhanced properties in high entropy oxides. Hexagonal manganites, RMnO3+δ (R = Sc, Y, Ho-Lu), can reversibly absorb large amounts of oxygen as interstitial ions below 400 °C, and are thus promising oxygen storage materials (OSMs) for oxygen separation, e.g. through chemical looping air separation (CLAS). Such applications require materials with a high reversible oxygen storage capacity (OSC), fast bulk ion transport and oxygen surface exchange rates. However, the oxidation kinetics of these materials are generally slow, with time to full oxidation on the scale of several hours. This implies a smaller available OSC when using short dwell times or fast heating and cooling rates, which are both necessary for efficient O2 gas production by cyclic processes like CLAS. Both the OSC and oxidation kinetics of RMnO3 are known to improve with larger R 3+ cations, either as the sole R-site cation or as a substituted element in YMnO3, or through aliovalent doping, e.g. with Zr 4+ on the R-site or Ti 4+ on the Mn 3+ site. Unfortunately, larger rare earth cations destabilize the desired hexagonal layered phase with respect to orthorhombic perovskite structure, which cannot accommodate interstitial oxygen. The Goldschmidt tolerance factor, t=(rR+rO)/2 ½ (rMn+rO), must be ≤ 0.855 (rR 3+ ≤ rHo 3+ ) to favor the hexagonal P63cm structure, while for larger rare earths like Dy 3+ , the P63cm hexagonal structure becomes metastable with respect to the orthorhombic Pnma perovskite structure. Several strategies have been demonstrated to extend the stability range of the hexagonal structure, like epitaxial strain, synthesis at high temperatures and low pO2 or prereduction of an amorphous gel in H2 atmosphere prior to crystallization. The two latter examples give a smaller tolerance factor due to reduction of a fraction of Mn 3+ to larger radius Mn 2+ , and the material is easily reoxidized at lower temperatures after high temperature synthesis.