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This study was conducted to determine the effects of different levels of oregano (Origanum vulgare) essential oil (OEO) on ruminal fermentation in lambs. Thirty-six weaned male Kivircik lambs were used as trial materials. Lambs were divided into one control and two treatment groups. For the two treatment groups, OEO was added into grower feed at a level of 250 and 500 ppm, respectively. Treatment period lasted for 9 weeks, including adaptation period. Feed and water was supplied ad libitum during the trial. Rumen fluid samples were collected from 9 lambs of each group, before the morning feeding and 3 and 6 hours after the feeding. From the ruminal fluid, pH, total volatile fatty acids (VFA), acetic acid, propionic acid, butyric acid and ammonia-nitrogen concentrations were determined. There were no effects of OEO on ruminal pH and concentrations of ammonia. In some of the rumen parameters (pH and total VFA), the results were statistically different periodically between groups and it was established that the time of measurement had an important role on the results of the rumen parameters. Furthermore, the quantitative increase of the total volatile fatty acid concentrations in the treatment groups according to the control was directly proportional to the increase of the oregano levels, in the periods except for the beginning. This situation makes us think that the oregano essential oil could have positive effects on digestion in lambs.