The orange and green arc bands of CaO are shown to arise from Ca + centered (4p,3d)-4s transitions analogous to the CaF A 2Π-X 2Σ+ and B 2Σ +-X 2Σ+ systems. The tremendous complexity of these Ca+O- systems is a consequence of the sixfold degeneracy of the O- p-hole, as compared to the nondegenerate F-(1S). The 0, 0 and 1, 1 bands of a D,d 1.3Δ-a 3Π transition are recorded by intermodulation spectroscopy and assigned (vibration, rotation, absolute parity) by nonstandard techniques. The D 1Δ state is found to lie below the isoconfigurational d 3δ1 state and D∼d spin-orbit perturbation causes the d 3Δ state to become irregular [3Δ2>3Δ 1>3Δ3). Two perturbation effects provide the key to the orbital interpretation of the orange and green arc bands: lambda doubling in a 3Π indicates that a π 4σσ3Σ+ state lies 1500 cm-1 above a 3Π D, d 1.3Δ∼B 1Π perturbations suggest recurrence of this 1500 cm-1 interconfigurational splitting which is the O-pσ/pπ splitting induced by the Ca+ ligand. © 1982 American Institute of Physics.