This paper presents a formulation of the optimal power flow (OPF) problem for transmission systems with the presence of wind generation. The OPF was modeled as a robust design optimization problem focused on ensuring that the probability that the power balance constraints are observed within a tolerance and that the boundaries of the design variables is respected. A simulation tool was developed implementing the model and it was tested with two power systems, two different wind probability distributions and two solution methods. Test results validated the proposed formulation giving optimal solutions that satisfy the power balance probabilistic constraint with the selected tolerance. The formulation is a simple model to calculate an optimal operation point for a power system with wind farms. This is an initial work in the formulation of an optimal operation model for large scale hybrid power system.