In this study, Ocimum basilicum essential oil was extracted according to hydrodistillation method and its antifungal activity (based on disk diffusion agar, well diffusion agar, minimum inhibitory concentration, and minimum fungicidal concentration) against fungi causing postharvest rots in apple was investigated. In addition, its constituents (by gas chromatography coupled to mass spectrometer), total phenol content, total flavonoids, and antioxidant activity (based on DPPH and ABTS free radical scavenging methods) were determined. Estragole was the main constituent of Ocimum basilicum essential oil with 65.43%. The essential oil contained 38.14 mg GAE/g total phenol and 11.17 mg QE/g total flavonoids. The results of antioxidant activity test showed that the free radical scavenging activity based on DPPH and ABTS methods were 66.85% and 73.70%, respectively. According to the results of disk diffusion agar and well diffusion agar methods, Penicillium expansum and Botrytis cinerea with the highest and lowest diameter of growth inhibition zones, respectively, were the most sensitive and resistant fungal strains to the essential oils. The results of minimum inhibitory and fungicidal concentration tests of essential oil showed a similar trend. In general, Ocimum basilicum essential oil contains many bioactive compounds with significant antioxidant and antifungal activity, which shows its ability as a natural preservative to improve the shelf life of horticultural products.