Ruminal protein degradability and intestinal rumen-undegraded protein digestibility (IDRUP) of whole-crop oat cut at: heading (H), milk (M) and milk-dough (M-D) stages of maturity were measured. The experiment was conducted on three Holstein bulls fitted with ruminal and duodenal cannula using the in situ and mobile nylon bag methods. A significant effect of ensiling on rumen protein degradability parameters was shown. Ensiling increased both protein solubility (fraction A) and effective rumen degradability (ERD), decreased the content of potentially degradable fraction B (P < 0.001) and IDRUP (64.7 vs 55.3%; P < 0.001). The content of soluble fraction (A) and ERD increased, and the content of fraction (B) decreased from heading (H) to milk (M) stages. However, from M to M-D stages of growth, the content of fraction A (75.6 vs 70.5) and ERD (80.6 vs 78.2) decreased. Total tract protein digestibility decreased (P < 0.001) with increasing plant maturity. Moreover, IDRUP of M and M-D forages was lower than that of H forages (62.9, 56.3 and 56.6, respectively for H, M and M-D; P < 0.001). It can be concluded that in terms of protein availability from the gastrointestinal tract, the milk stage of growth seems to be the optimal harvest time.