Context: Oat (Avena sativa L.) serves as a crucial forage crop, but its production in arid regions, particularly in the Hexi Corridor of Northwest China, is often constrained by inefficient nitrogen (N) use and low quality. Overapplication of N and inconsistent mowing schedules contribute to low N use efficiency (NUE) and suboptimal aboveground dry matter and quality. Objective: This study hypothesized that optimizing N application rate and mowing frequency could enhance forage nutritive value while improving NUE. Methods: A four-year field experiment was conducted using subsurface drip fertigation with four N application rates (0, 90, 180, and 270 kg N ha-1) and four mowing frequencies: M1 (milky stage), M2 (jointing and milky stages), M3 (jointing, booting and milky stages), and M4 (jointing, booting, grain-filling, and milky stages). Results: Results showed that N application, mowing frequency (M) and their interaction (N x M) significantly affected dry matter, regeneration, crude protein (CP) content and NUE (P < 0.05). Dry matter increased with N application, while CP plateaued at 180 kg N ha-1 . No significant differences in aboveground dry matter or CP were observed between 180 and 270 kg N ha-1 (P < 0.05). NUE declined linearly with increasing N application rates. Dry matter and CP followed a unimodal pattern with mowing frequency, peaking at M3. The combination N180M3 produced the highest dry matter and CP over four years. Additionally, accumulated dry matter and CP increased with effective cumulative temperature, while NUE showed a quadratic relationship. Conclusions: Applying 180 kg N ha-1 and mowing at jointing, booting and milky stages provided the best balance of aboveground dry matter, quality, and NUE. Significance: Appropriate mowing frequency and nitrogen application improved growth and quality of oat.