In this paper we first develop an analytic closed-form model for the failure probability (FP) of a large digital circuit due to gate oxide breakdown. Our approach accounts for the fact that not every breakdown leads to circuit failure, and shows a 6-11x relaxation of the predicted lifetime with respect to the ultra-pessimistic area-scaling method. Next, we develop a posynomial-based optimization approach to perform gate sizing for oxide reliability in addition to timing and area.