Rotational spectra of carbon-chain molecules C5O, C7O, and C9O have been observed for the first time by using a Fabry-Perot type Fourier transform microwave (FTMW) spectrometer cooperated with a pulsed discharge nozzle (PDN). The molecules have been generated by the discharge of C3O2 diluted in Ar. The spectra consist of series of R-branch transitions typical of linear molecules in singlet electronic states. For C5O, transitions for all the possible singly substituted C-13 and O-18 isotope species have also been detected in natural abundance. These measurements with an assumption of linearity have enabled a complete substitution structure to be derived : r(s)(C-(1)-O) = 1.1562(11) Angstrom, r(s)(C-(2)-C-(1)) 1.2552(30) Angstrom, r(s)(C-(3)-C-(2)) = 1.2881(38) Angstrom, r(s)(C-(4)-C-(3)) = 1.2947(21) Angstrom, and r(s)(C-(5)-C-(4)) = 1.2736(10) Angstrom. The CC bond lengths in C5O were found to be much more uniform than those in an isoelectronic molecule HC5N. The canonical structure which reproduces observed internuclear distances in C5O has been suggested. By assuming the same partial structure as that in the C5O molecule, the averaged bond lengths for the rest of C-C bonds in C7O and C9O have been determined to be 1.270 and 1.274 Angstrom, respectively. Along with the recent results of tripler CnO (n = 2, 4, 6, and 8) [Ohshima, Endo, and Ogata, J. Chem. Phys. 1995, 102, 1493-1500], the CC bonds in CnO are found to become more uniform cumulene-type double bonds with increasing n and to converge to a constant length.