Our previous theoretical studies on all-carboatomic C 18 showed that, due to the special sp-hybrid form of its carbon atoms, the molecule exhibits an unusual in-plane π electron system (π in ) in addition to the out-of-plane π system (π out ) that are common in conjugated molecules . It can be expected that the cyclocarbon oxides should possess the special electronic structure that interests us like C 18 . In addition, it is a meaningful topic to explore how the presence of -CO groups affects the electron delocalization and aromaticity of the cyclocarbon oxides. So, in this work, the bonding character and electron delocalization of three C 18 precursors, C 18 -(CO) n (n = 6, 4, and 2), are analyzed in detail using a variety of wavefunction analysis methods based on reliable quantum chemistry calculations. Their respective aromatic characteristics will be discussed and compared in terms of the number of -CO groups.