Covalent organic frameworks (COFs) have been developed as metal-free catalysts for oxygen reduction reaction (ORR) due to their well-defined catalytic sites and pores. While heteroatom doping is a common strategy to modulate catalytic properties, the role of low polar, steric substituents such as methyl groups remain largely unexplored due to their minimal direct electronic effect. Herein, we systematically investigate the influence of methyl groups in COFs on ORR catalysis by precisely controll