High-yield dairy farms have maximised their productivity, which is a challenge concerning animal physiology even beyond the transition period. While several studies have verified a wide range of beneficial effects of plant essential oils during the periparturient period in dairy cows, their effects in mid-lactation are less studied. However, this period is critical for maintaining milk production and metabolic stability in high-yielding cows. The objective of the present study is to determine the effects of supplementation with herbal plants on immunity and antioxidant status in mid-lactation dairy cows. Thirty-six Holstein-Friesian cows used in this experiment were randomly assigned to 6 groups (1 control and 5 treatments, n = 6/group). The treatments were conducted using oregano, basil, thyme, rosemary, or yarrow, while the control was exempt from any supplementation. The feeding experiment lasted 14 days. Milk samples were collected on days 1 and 14 to assess milk quality. Milk and blood samples for molecular analysis were collected on day 15, following the 14-day feeding trial. The applied herbs were standardised to two grams of essential oil/cow/day. All parameters were analyzed by a linear mixed model and expressed relative to the control group. The basil essential oil resulted in a significant increase in the total immunoglobulin level in milk, and the serum IgM level was found to be significantly higher in the rosemary. Significantly higher serum total antioxidant capacity (TAC) values were detected in the oregano, thyme, rosemary, and yarrow groups. Significant increases were observed in milk TAC for groups given oregano and thyme. The activity of the superoxide dismutase enzyme (SOD) was found to be significantly higher in the oregano group, and significantly lower C-reactive protein levels, an indicator of inflammation and oxidative stress, were measured in the oregano and rosemary groups. Of the antioxidant-related genes, a significantly higher expression of the SOD gene was detected in the oregano group, while for the glutathione peroxidase gene, significantly higher mRNA expression was observed in the oregano and thyme groups. In conclusion, our results suggest that dietary supplementation with oregano and thyme improves antioxidant status in mid-lactation dairy cows. These findings may support the development of more precisely tailored feeding protocols to enhance well-being and promote the health and performance of dairy cows during mid-lactation.