In limited growing conditions, intercropped field peas and oats can represent a significant source of forage rich in protein. If applied correctly, factors such as nitrogen fertilizer, the mowing phase, and sowing norms can significantly increase the productivity of these mixtures. Field trials were conducted to examine their productivity under different nitrogen levels (0, 40, 80 kg ha⁻¹), different sowing norms/mixtures (field peas: oats—100:15%; 100:30%), and two stages of growth (full flowering, full pod formation). Nitrogen fertilizer and different sowing norms had a significant effect on the biomass, hay, and crude protein yields. On average, the highest hay yields were achieved with 80 kg ha⁻¹ N (4.96 t ha⁻¹), followed by 40 kg ha⁻¹ N (4.27 t ha⁻¹). The highest protein yields were achieved with 40 kg ha⁻¹ N (CP—704.1 kg ha⁻¹), followed by 80 kg ha⁻¹ N (CP—637.6 kg ha⁻¹). Sowing norm 100:30% achieved higher hay yields: 100:30%—4.82 t ha⁻¹; 100:15%—4.44 t ha⁻¹, while 100:15% achieved higher crude protein yields: 100:15%—730.4 kg ha⁻¹; 100:30%—692.7 kg ha⁻¹ on average. The costs were not significantly increased with the nitrogen fertilizer, but the net profits were increased by as much as 163%, depending on the nitrogen level and the mixture. Nitrogen fertilizer also achieves higher economic efficiency for the mixture 100:15% compared to the 100:30% mixture. Mixtures of field peas and oats outperform single-grown crops and provide cost-effective feed for a short time. Using optimal seed ratios and nitrogen fertilizer can significantly increase the productivity and profitability of the feed with minimal impact on the overall production costs.