ABSTRACT The goal of this study was to evaluate the impact of feeding oregano (Origanum vulgare) leaves extract on metabolic , performance and health of lactating buffalos, blood constituents, digestibility, milk production, and milk composition of dairy buffalos. Twenty dairy buffalos were chosen at random to assign treatments: control, 10, 20, and 30 g/head/day of oregano leaves extract as CON., OR1, OR2, and OR3, respectively. Results showed that the addition of oregano leaves extract had a significant effect (P<0.05) on all parameters of digestion coefficients DM, OM, NFE, CP, CF, EE, DCP, and TDN. The addition of Oregano leaves extract  (OR2 and OR3)  substantially raised the digestibility coefficient of CP and CF (P<0.05), with the inclusion of 20 and 30 g/head/day oregano leaves extract  yielding the highest values. OR2 and R3 supplemented with 20 and 30 g/head/day oregano leaves extract had the best total digestible nutrients (TDN) and digestible crude protein (DCP). The groups (OR2 and OR3) showed significantly (P<0.05) the highest milk yield of both actual and 4% fat-corrected milk, followed by group (OR1), while the control group (CON.) had the lowest milk yield. Animals fed 20 and 30 g/head/day Oregano leaves extract  recorded significantly (P<0.05) better values of feed conversion ratio (FCR) compared with control. The experimental groups showed more significant (P<0.05) variations in the milk content of protein, fat, TS, SNF, lactose, and ash than the control group when supplemented with oregano leaves extract. Supplementation with 30 g/head/day oregano leaves extract  decreased serum total protein, albumin, urea, creatinine, cholesterol, LDL, VLDL, and triglyceride concentrations (P<0.05) compared with other groups, without difference (P > 0.05) among groups in ALT and AST. The average value of serum globulin was significantly higher in oregano leaves extract groups than the control group. The obtained results demonstrated that supplementation groups also tended to increase glucose value as the BHBA and NEFA levels decreased (P > 0.05) that protected against ketosis.