The purpose of this study was to examine the effects of supplementing milk replacer (MR) with aromatic oregano (Origanum onite L.) water on weaning age, performance during the pre- and post-weaning periods, immune systems, blood parameters and general health of 40 Holstein Friesian calves. The calves fed with oregano water (OW)supplemented MR were weaned earlier than those in the Control group. There were no statistical differences in the liveweight and body measurements among the groups at the weaning age and post-weaning period. The feed intake of the calves fed with OW was higher than that of calves in the Control group in the pre-weaning period. The OW supplementation significantly increased the faecal firmness of calves (P < 0.05). It also decreased the number of E. coli, Coliforms and Enterobacteriaceae pathogens, but only the amount of Enterobacteriaceae was significantly decreased (P < 0.05). Non-pathogens, such as Lactic acid bacteria, were not affected by OW supplementation. The OW supplementation had no significant effect on hematologic and biochemical blood values. There was significant interaction between age and groups in gamma-glutamyl transpeptidase (P < 0.05). The immunoglobulins (IgA, IgG and IgM) values of calves were steadily higher than those of the calves in the Control group. The general health profiles of the calves improved. It can be concluded that MR containing 1-2% OW fed to calves in the early weaning period is a promising treatment to improve their wellbeing and shorten the weaning period.