This study was conducted to examine in vivo long-term effects of dietary dried oregano (Origanum vulgare ssp. hirtum) whole plant on rumen fermentation, enzyme profile and microbial communities. For this purpose, eight healthy, adult, non-lactating Alpine goats were kept in tie stalls equipped for individual feeding and randomly divided into two homogeneous groups: one fed 0.6 kg of a concentrate mixture and 0.6 kg of wheat straw without any supplementation and served as control group (CON) while the other group (OR) fed the same diet of CON but supplemented with 20 g of dried oregano plants (OPs) to provide daily dosage of 1 ml of essential oil (EO) per animal. The experimental period lasted 69 days and individual rumen fluid samples were obtained every 2 weeks at 0 and 4 hr after feeding. The results showed that dietary supplementation with OPs increased the protease activity (p < .001) and ammonia concentration (p < .05) in the rumen. Among the studied microbial populations, Peptostreptococcus anaerobius (p = .028) and Clostridium sticklandii (p < .001) were found to be the most sensitive to oregano at the current dosage. Furthermore, the total methanogen population significantly decreased (p < .05). It is concluded that a long-term dietary administration of OPs can suppress specific rumen micro-organisms and modify rumen fermentation favourably at least by means of suppressing methanogens.