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Black tea is one of the most popular beverages consumed in the world. It is stronger in taste as well as in flavour compared to other less oxidized teas. It is made from the leaves of the shrub Camellia sinensis var. assamica. Black tea can be supplemented with other plant parts to enhance its flavour and health-promoting properties. In India, Ocimum spp. leaves have been used for their medicinal properties since ancient times. These leaves can be added during black tea preparation to enhance their aroma and healing activities. O. gratissimum, known as "Scent Leaf", is traditionally used for the management of many diseases, such as the common cold and cough. This work was designed to evaluate the antioxidant interaction between black tea and O. gratissimum (leaves) at five different ratios (1:1, 1:2, 1:3, 2:1, and 3:1). To determine the antioxidant activity, chemical-based methods and ex vivo assays were conducted. The total phenolic and flavonoid contents were calculated by Folin's reagent and aluminium chloride colorimetric assays, respectively. The antioxidant interactions were determined by the combination index (CI), using CompuSyn software. The black tea exhibited higher radical quenching activity (DPPH, ABTS, and NO) and antihaemolytic and anti-lipid peroxidation potential compared to the Ocimum gratissimum infusion. Variation in the antioxidant capability was observed for various ratios of the black tea and O. gratissimum (BT+OG) combination. The antioxidant interaction between BT and OG ranged from nearly additive to antagonistic. The total phenolic content was higher for O. gratissimum, whereas the total flavonoid content was high in black tea. The binary mixture of BT+OG at all ratios (3:1, 2:1, 1:1, 1:2, and 1:3) expressed similar phenolic and flavonoid levels. Overall, black tea and O. gratissimum displayed additive antioxidant interaction and the highest free radical scavenging potential at a 3:1 proportion in all the performed parameters. [ABSTRACT FROM AUTHOR]