The electronic structure of the CuO2 planes and CuO3 chains in single-domain crystals of YBa2Cu3Ox has been investigated as a function of oxygen concentration (6≤x≤7) using polarization-dependent x-ray-absorption spectroscopy of the O 1s and Cu 2p core levels. The polarization-dependent observation of unoccupied states with O and with Cu orbital character parallel to the a, b, and c axes of the crystals allows the determination of the number of hole states in Cu 3d2x-y2 and O 2px,y orbitals in the CuO2 planes as well as in Cu 3d2y-z2 and O 2py,z orbitals in the CuO3 chains. States with Cu 3d23z-r2 orbital character contribute less than 10% to the total number of states near the Fermi level. The number of holes in the planes and in the chains is found to be correlated with the superconducting transition temperature.