Nanocarbon can promote robust and efficient electrocatalytic water oxidation through active surface oxygen moieties. The recent mechanistic understandings (e.g., active sites) of metal-free carbon catalysts in oxygen evolution reaction (OER), however, are still rife with controversies. In this work, we describe a facile protocol in which eight kinds of aromatic molecules with designated single oxygen species were used as model structures to investigate the explicit roles of each common oxygen group in OER at a molecular level. These model structures were decorated onto typical nanocarbon surfaces like onion-like carbons (OLC) or multiwalled carbon nanotubes (MWCNT) to build aromatic molecule-modified carbon systems. We show that edge (including zigzag and armchair) quinones in a conjugated π network are the true active centers, and the roles of ether and carboxyl groups are excluded in the OER process. The plausible rate-determining step could be singled out by H/D kinetic isotope effects. The turnover frequency per C═O (∼0.323 s-1 at η = 340 mV) in 0.1 M KOH and the optimized current density (10 mA/cm2 at 1.58 V vs RHE) of quinone-modified carbon systems are comparable to those of promising metal-based catalysts.