This experiment evaluated the ruminal digestibility of Turkish oregano leaves in dairy buffalo and cows. Ruminally-cannulated, multiparous Brown Swiss cows (n = 3) and water buffalo (Bubalus bubalis; n = 3) were used in the experiment. The ad libitum basal diet was balanced to NRC requirements for a dry, nonpregnant multiparous dairy cow (680 kg) and consuming 12.8 kg of DM/d. Air-dried, ground, weighted oregano leaves were inserted in the rumen of all animals before the morning feeding with heat-sealed nylon bags ( × 4 replicates). After incubation (4, 8, 24, 48, 72 h), dry matter (DM), crude protein (CP), aNDFom, and ADFom concentrations were determined and compared with initial leaf composition. In situ disappearance (ISD) parameters were evaluated using PROC MIXED of SAS. Degradability kinetic parameters were fit with an exponential model using PROC NLIN and differences in parameters between the species were determined by t test. The means for ISD (%) of DM, CP, aNDFom, and ADFom were 50.9 ± 1.9 vs. 61.0 ± 2.3; 31.7 ± 3.5 vs. 62.6 ± 2.7; 41.0 ± 4.2 vs. 41.2 ± 3.5; and 49.4 ± 2.2 vs. 60.5 ± 2.4 for cows and buffalo, respectively, after 24 h incubation. Mean ruminal disappearance of DM, aNDFom and ADFom did not differ between the species, but CP degradation showed a species effect (P = 0.04) and a species × time interaction (P = 0.04). The degradation rate (% h⁻¹) constants of DM, aNDFom, ADFom, and CP were 5.24 ± 0.17 vs. 4.83 ± 0.80; 4.90 ± 0.69 vs. 3.63 ± 1.29; 4.43 ± 0.68 vs. 4.22 ± 0.79; and 2.40 ± 0.80 vs. 3.35 ± 0.97 for cows and buffalo, respectively. The immediately-degraded fractions of DM and CP were significantly greater in buffalo than the cows (27.1 ± 1.5 vs. 17.8 ± 2.1% of DM, P = 0.02 and 32.1 ± 0.9 vs. 13.6 ± 6.3% of CP, P = 0.04, respectively), but the other model parameters showed no differences between the species. In conclusion, CP of oregano leaves is degraded more quickly in the rumen of dairy buffalo than cows, although extent of digestion did not differ after 72 h of incubation. The ruminal digestibility of Origanum onites L. leaves was similar to that of forages commonly fed to dairy cattle.