We have performed semiempirical tight-binding calculations of the electronic structure of YBa2Cu3O7, with dand s orbitals included for all the metal atoms and p and sorbitals for oxygen. Here we report studies of oxygen vacancies on the O(1) chain sites in YBa2Cu3O7-y. The modification of the density of states ρ(E) and the shift of the Fermi energy Ep were calculated for 0 < y ≤ 1.0. The Fermi energy is found to increase monotonically with y, confirming the expectation that oxygen vacancies act as donors. Also, ρ(EF)is found to decrease with y.