Electron emission from molybdenum field emission microcathode arrays is sensitive to the contamination of the emitters by the residual gases. We studied the effects of O-2 exposures on the emission properties of several Spindt type molybdenum field emitter arrays. The arrays were exposed to 0-10000 L of O-2 and the resulting changes in electron emission characteristics were measured. Exposure of the field emitters to O-2 resulted in emission degradation ranging from 6.2 +/- 1.3% for 10 L to 99.2+/-0.4% for a 10000 L exposure. The emission is recoverable for low exposures, but exposures beyond 1000 L result in permanent loss in emission current. We found that the degradation effects are similar in both the de and pulsed modes of operation of the device. Therefore, de mode testing can be used as an effective acceleration method in establishing the device lifetimes under various vacuum conditions.