We predict the adiabatic electron affinities of C3O and C4O based on electronic structure calculations, using a large triple-zeta basis set with polarization and diffuse functions (TZ2Pf+diff) with the SCF, CCSD, and CCSD(T) methods as well as with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Our results imply electron affinities for C3O and C4O; EA(C3O) = 0.93 eV + 0.10 and EA(C4O) = 2.99 +/- 0.10. The EA(C3O) is 0.41 eV lower than the experimental value of 1.34 +/- 0.15 eV, while the EA(C4O) is 0.94 eV higher than the experimental value of 2.05 +/- 0.15 eV. Optimized geometries for all species at each level of theory are given, and harmonic vibrational frequencies are reported at the SCF/TZ2Pf+diff and CCSD/aug-cc-pVDZ levels.