Background: Ocimum gratissimum L. (OG) has medicinal importance, especially in the treatment of epilepsy, fever, diarrhea, mental illness, etc., Objectives: The present study focused on evaluation of the total phenolic content, total flavonoid content, and reducing and antioxidizing activities from methanolic extract of different plant parts, namely leaves, stem, roots, and flowers of OG. Materials and Methods: The quantification of phenolic and polyphenolic compounds was assessed by high-performance liquid chromatography (HPLC). Results: The results revealed that the root extract showed the highest phenolic content (323.93 ± 2.062 μg/mg) and reducing property (150.57 ± 1.76 μg/mg), whereas the highest flavonoid content was found in leaves 72.09 ± 1.269 μg/mg. The highest radical scavenging activity (68.12% ± 0.527%) was observed for the root extract in 2, 2-diphenyl-1-picrylhydrazyl method. Similar significant results were observed by 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) method as well for all investigated extracts. HPLC–diode-array detector analysis was done to find the secondary metabolite composition of the methanol extracts which lead to the characterization of different flavonoids and phenolic acids in OG extracts. Conclusion: These results showed that this species can be a source of phenolic compounds and antioxidants, thereby suggesting it for consumption as a source of natural food antioxidant to help hinder against various detrimental effects of reactive species-induced ailments.