We report the structural and electronic properties of ordered ZnS_1-xO_x alloys calculated in various structures (CuAu-I, Cu₃Au, Luzonite and Famatinite) using a first-principles total-energy calculations based on the hybrid full-potential augmented plane-wave plus local orbitals (APW+lo) method. We have used the local-density approximation (LDA) for the exchange and correlation potential. The calculated gaps range from 0.97 for O-rich to 1.55 eV for S-rich ZnS_1-xO_x systems. We predict that ordering affects significantly the electronic band structures. The alloys in the CuAu-I structure are found to have direct smaller band gap of 0.49 eV. The origin of the reduction in the band gaps is discussed, as well as the effects of increasing Oxygen in ZnS_1-xO_x alloys. These alloys whose constituents are rather size mismatched, a strong reduction of the band gap is found. Beside the presence of lattice mismatch, the Zn 3d electrons effects are found to play an important role in the determination of the electronic structure. Their effect on the gap, and also on the k dispersion in the valence bands is significant. We have identified the microscopic origin of the main features in the band structures and found that not only ordering but also Zn 3d electrons effects are responsible of the reduction of the band gap in these alloy systems.