In fact, both species are quite confusing to each other.
Although they may be confused, Leaves are usually shorter than 2.5 cm narrower than 1.5 cm in O. americanum, always longer than 2.5 cm and broader than 1.5 cm in O. basilicum. Corolla 3-5 mm long in former, 6-9 mm long in latter. fruiting calyx 4-5 mm long in former, 5-7 in latter. Both are herbs with fruiting calyx ascending or spreading, upper calyx lobe circular with wings decurrent (joined) to middle of calyx, and front two teeth of calyx quite distinct.