Olive tree cultivation is a vital activity in central and southern Tunisia's semi-arid and arid regions due to their nutritional value and adaptability to water-stressed conditions. In this study, the phytochemical profile, including the content of α-tocopherol, minerals, polyphenols, flavonoids, and tannins, was identified from the leaves of two oleaster trees from the “Chemlali” variety in southern Tunisia. Two olive sites, rainfed and irrigated, were used for comparison. Antioxidant activities were assessed using the 2,2-diphenylpicrylhydrazyl (DPPH) assay. HPLC/MS analysis of ethyl acetate leaf extracts identified 15 organic compounds. The main phenolic compounds were quinic acid (569.11±1.49 mg/100 g DW and 560.95±0.80 mg/100 g DW in rainfed and irrigated leaves, respectively) and luteolin-7-O-glucoside (127.36±4.61 and 141.64±0.45 mg/100 g DW, respectively). The α-tocopherol, sugar, and mineral contents were also evaluated. DPPH scavenging capacity was slightly higher in rainfed leaves (90%) than in irrigated ones (88%). A significant decline in total phenolics and sugar content was observed in leaves from irrigated trees compared to those from the rainfed site. The sugar content ranged between 8.4 g/kg in the irrigated trees and 7.54 g/kg in the rainfed ones. The minerals K, Mg, Na, and Ca were detected in notably high quantities in the extracts of both sites. Overall, the effects of irrigation were very pronounced, mainly in terms of sugar content and total phenolic compounds. Olive tree leaves consistently serve as an outstanding source of high-value natural compounds with significant biological activities, even though irrigation directly influences leaf composition and quality.