Many technologies require oxides with high electronic conductivity or mobility (e.g., transparent conducting oxides, oxide photovoltaics, or photocatalysis). Using high-throughput ab initio computing, we screen more than 4000 binary and ternary oxides to identify the compounds with the lowest electron effective mass. We identify 74 promising oxides and suggest a few novel potential n-type transparent conducting oxides combining a large band gap to a low effective mass. Our analysis indicates that it is unlikely to find oxides with electron effective masses significantly lower than the current high-mobility binary oxides (e.g., ZnO and In2O3). Using the large data set, we extract chemical rules leading to low electron effective masses in oxides. Main group elements with (n-1)d10ns0np0 cations in the rows 4 and 5 and groups 12-15 of the periodic table (i.e., Zn2+, Ga3+, Ge4+, Cd2+, In3+, Sn4+, and Sb5+) induce the lowest electron effective masses because of their s orbitals hybridizing adequately with oxygen. More surprisingly, oxides containing 3d transition metals in a low oxidation state (e.g., Mn2+) show also competitive effective masses due to the s character of their conduction band. (Graph Presented).