Three-dimensional potential energy functions have been generated for the A2Sigma + electronic state of C2O- using highly correlated electronic multi-reference configuration interaction wavefunctions. These were employed in variational calculations of the rovibrational states. The rotational constant has been calculated as Be = 0.3960 cm-1, with RCOe = 1.240 Å and RCCe = 1.244 Å. Rovibrational levels for J = 0 and 1 are reported for energies up to about 4 800 cm-1. Only weak anharmonic resonances are found in this energy region. The theoretically predicted A2Sigma + -X2 absorption spectrum is found to be in excellent agreement with the observed one in a neon matrix.