Phenolic antioxidant intake is encouraged to prevent oxidative damage, and antioxidant synergy is considered an advantage in adding polyphenols from varied plants. This study investigated the antioxidant and synergistic interactions among olive leaf (OL), bay laurel (BL), and rosemary (RL) leaf powder mixture (LPM: OL + BL + RL), using in vitro chemical tests [TPC, ORAC, TEAC-ABTS, FRAP
combination index (CI)], and in vivo validation on blood oxidative status, metabolic profile, and intestinal histomorphology in laying hens. The in vitro study indicated a whole higher antioxidant capacity for the LPM than respective single/double-leave combinations. The LPM CI value (IC50, 0.60) indicated a synergistic effect compared to the binary combinations. Thus, the LPM was validated in vivo through dietary supplementation on sixty Lohmann Brown hens (30 weeks old), reared in an indoor-outdoor rearing system divided. The hens were allocated into two experimental groups (n. 30): basal control diet group
and diet supplemented group with 6 g/kg feed of LPM) containing OL, BL, and RL (respectively, at 65.7%:18.9%:15.4%), for 60 days. The LPM improved (