Although concentrate feeds supply most carbohydrates and proteins, the protein component represents the most expensive fraction. A substantial portion of concentrate protein can be replaced with more economical protein sources from forages, particularly from legumes such as pea (Pisum sativum ssp. arvense L.), combined with cereals like oat (Avena sativa L.). Mixtures of these annual legumes and cereals generate a synergistic effect, where oats contribute yield stability and energy, while peas enhance protein concentration and improve forage preservation quality. Assessing protein quality requires understanding the distribution of individual protein fractions classified according to the Cornell Net Carbohydrate and Protein System (CNCPS), which categorizes proteins from PA (non-protein nitrogen) to PC (undegradable proteins bound to lignin, tannins, or Maillard products). This study investigated the influence of pea–oat seed ratios—SR (80:20, 60:40, 40:60, and 20:80) and developmental stages—S (early flowering and pod filling) on the dynamics of protein fractions in green biomass. Results showed that soluble protein fractions (PA, PB₁) decreased during maturation due to nitrogen translocation to developing grains, while structural and undegradable fractions (PB₂, PB₃, PC) increased, particularly in mixtures with higher oat proportions. The 60:40 pea:oat ratio produced the most balanced protein profile, maximizing the proportion of moderately degradable proteins (PB₂), which are crucial for efficient microbial protein synthesis in the rumen. This ratio also optimized the synergy between legume nitrogen fixation and cereal energy supply, enhancing sustainable ruminant nutrition. Statistical analysis confirmed significant differences between growth stages and mixture compositions. Overall, pea–oat mixtures represent a key component of economically viable and ecologically sustainable forage production for ruminant livestock systems.