As device sizes shrink and current densities increase, the probability of device failures due to gate oxide breakdown (OBD) also increases. To provide designs that are tolerant to such failures, we must investigate and understand the manifestations of this physical phenomenon at the circuit and system level. In this paper, we develop a model for operational OBD defects, and we explore how to test for faults due to OBD. For a NAND gate, we derive the necessary input conditions that excite and detect errors due to OBD defects at the gate level. We show that traditional pattern generators fail to exercise all of these defects. Finally, we show that these test patterns can be propagated and justified for a combinational circuit in a manner similar to traditional ATPG.