Onosma species have traditionally been used as laxative and anthelmintic agents as well as their uses in eye, blood diseases, bronchitis, abdominal pain, strangury, thirst, itch, leucoderma, fever, wounds, burns, piles, and urinary calculi. This study examines the investigation of biological activities of ethyl acetate, methanolic, and aqueous extracts from Onosma gigantea Lam. The extracts were evaluated for the antioxidant and enzyme inhibitory activities. Phenolic compounds in the extracts were also detected by RP-HPLC-DAD. The aqueous extract showed remarkable antioxidant activity in phosphomolybdenum [134.31 mu mol trolox equivalents (TEs)/g air dry matter (adm)], chelating effect [32.97 mu mol ethylenediaminetetraacetic acid (disodium salt) equivalents (EDTAEs)/g adm], radical scavenging [on DPPH (32.14 mu mol TEs/g adm) and ABTS (58.68 mu mol TEs/g adm)), and reducing power [CUPRAC (50.23 mu mol TEs/g adm) and FRAP (40.96 mu mol TEs/g adm)] assays. The aqueous extract also exhibited considerable inhibitory activity of tyrosinase (10.48 mu mol kojic acid equivalents/g adm) and alpha-glucosidase [1320.53 mu mol acarbose equivalents (ACES)/g adm]. It has been determined that the methanolic extract showed significant AChE (31.57 mu mol galanthamine equivalents (GALAEs)/g adm) and alpha-amylase (6.75 mu mol ACEs/g adm) inhibitory activity. The ethyl acetate, which contains low polarity compounds, showed the highest BChE inhibitory activity (6.87 mu mol GALAEs/g adm). In general, it is found that there is a positive correlation between the phytochemical contents of the extracts and their biological activities. The results indicate that O. gigantea extracts could be considered as a novel source of bioactive agents with functional connections.