Mono- and polycyclic valence isomers of carbo-[3]oxocarbon C9O3 and carbo-[5]oxocarbon C15O5 have been characterized on the singlet spin state potential energy surface. By contrast to their geometry, their relative stability is highly sensitive to the calculation level. The performance of LDA, GGA, meta-GGA, and hybrid functionals is compared to that of HF, post-HF, and multiconfigurational calculations. The results obtained for C9O3 are compared to those obtained for hydrocarbon analogues such as [3]pericyclyne C-9(H-2)(3) and carbo-[3]radialene C-9(CH2)(3) and are analyzed on the basis of an energy decomposition scheme. The respective role of the exchange and correlation counterparts of the functional in the discrepancy of the results is discussed.