We study theoretically the thermopower in cobalt oxides generalizing the Heikes formula. In the oxides, states of carriers are largely degenerate because the crystalline field and Hund's rule coupling compete in cobalt ions. It is shown that the degeneracy, together with strong correlation of $3d$ electrons, causes the large thermopower. The recent experiments in ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ are discussed in the light of the theoretical result.