Nitrogen fertilization is a routine part of forage management strategies for grasses, but the effects on forage nutritive value have been inconsistent. Our objectives were to evaluate the effects of N fertilization on the nutritive value of fall-grown oat fertilized at planting with 20, 40, 60, 80, or 100 kg N/ha of urea or 2 rates of dairy slurry (42,300 or 84,600 L/ha). Oat forages fertilized with urea or dairy slurry had greater (P < 0.001) concentrations of fiber components compared with those harvested from unfertilized check plots (0 kg N/ha), and fiber concentrations increased linearly (P < 0.001) with urea fertilization rate. In contrast, concentrations of water-soluble carbohydrates were greatest for unfertilized forages (21.2%), but declined linearly (P < 0.001) with urea fertilization, exhibiting a minimum of 13.5% at the 80 kg N/ha application rate. Similarly, non-fiber carbohydrate also declined linearly (P < 0.001) from 34.8% for unfertilized plots to a minimum of 24.6% at the 80 kg N/ha urea application rate. Expressed on a percentage of DM basis, fertilization with urea resulted in linear (P < 0.001) increases in crude protein (CP), neutral-detergent soluble CP, neutral-detergent insoluble CP, and acid-detergent insoluble CP, but effects on subcomponents of the total CP pool were not observed when concentrations were expressed on a percentage of CP basis (P > 0.117). The summative calculation of energy (TDN) was closely related to N fertilization rate during both the 2013 (Y = −0.038 x + 72.2; r² = 0.961) and 2014 (Y = −0.040 x + 69.2; r² = 0.771) production years. Following 30- or 48-h incubations in buffered rumen fluid, in vitro DM disappearance was greater (P ≤ 0.024) for unfertilized forages compared with those fertilized with either urea or dairy slurry, and DM disappearance declined linearly (P ≤ 0.001) with urea fertilization rate; however, these responses were not detected (P ≥ 0.109) for neutral-detergent fiber disappearance. Overall, the forage nutritive value of fall-grown oat declined mildly in response to N fertilization, resulting in losses of approximately 0.4 percentage units of TDN for every 10 kg N/ha applied as urea.