In the cluster approach, we consider the peculiarities of charge-transfer (CT) states and CT O2p -> Cr 3d transitions in the octahedral (CrO6)(9-) complex. We have computed the reduced matrix elements of electric-dipole transition operator on many-electron wave functions - the initial and final states of CT transitions. We have parameterized the obtained results and computed the relative intensities of various allowed CT transitions in the absence of the mixing of CT configurations having the same symmetry. Using the Tanabe-Sugano technique, we have taken into account this mixing and obtained the energies of many-electron CT transitions and their actual intensities as well. We have also allowed for the Coulomb interaction between the 2p-electrons of the O2- ligands and the 3d-electrons of the central Cr3+ ion in the (CrO6)(9-) cluster. This interaction proved insignificant for the optic spectra. Modeling the optic spectrum of chromium-based oxides has yielded a complicated CT band consisting of 33 lines with the main maximum at about 7 eV and satellites in the range of 4-5 and 8-9 eV. The total extent of the CT band is about 8 eV. The model spectrum is in satisfactory agreement with experimental data, which shows the limited validity of the generally accepted notion of a simple structure of CT spectra.