Malaysia is ranked as the second largest oil palm producer in the world after Indonesia. This leads to the generation of large quantities of oil palm (Elaeis guineensis Jacq.) leaves (OPLs) annually, considered an underutilized oil palm biomass with low economical value. The present study aimed to study the effects of several parameters of ultrasound-assisted extraction (UAE) and the acid hydrolysis of phenolic compounds from OPLs using the single factor experimental approach. The effects of different solvents (hexane, ethyl acetate, 1:1 methanol–ethyl acetate, absolute methanol, and 4:1 methanol–water), solid–liquid ratios (1:20, 1:40, 1:50, 1:60, and 1:70), times (0.5, 1, 2, 3, and 5 h), and temperatures (25, 30, 40, 60, and 70 °C) were investigated for UAE. Moreover, the effects of acid concentration (0.2, 1, 2, 4, 6, and 12 M), incubation time (10, 30, 45, 60, and 90 min), and incubation temperature (65, 75, 85, 90, and 95 °C) on the acid hydrolysis of flavonoid glycosides were also determined. The results revealed that some of the tested parameters had prominent effects on the total phenolic (TPC) and total flavonoid (TFC) contents, as well as the DPPH free radical scavenging activity recovered from the OPLs. The optimal UAE conditions were determined to be 0.5 h at 25 °C using 4:1 methanol–water and 1:50 solid–liquid ratio, producing OPL extracts with TPC and TFC at 335.30 and 60.67 milligrams quercetin equivalents per gram of extract (mg QCE/g extract), respectively, and DPPH free radical scavenging activity at 94.06%. The phenolics present in OPLs were optimally hydrolysed using 6 M hydrochloric acid with an incubation period of 45 min at 95 °C with TPC, TFC, and DPPH free radical scavenging activity at 126.33 milligrams gallic acid equivalents per gram of extract (mg GAE/g extract), 36.08 mg QCE/g, and 54.88%, respectively. Moreover, acid hydrolysis managed to optimally recover the total apigenin content (TAC), total luteolin content (TLC), and total flavonoid C-glycoside content (TFCGC) with values of 79.12 micrograms vitexin equivalents per milligram of extract (µg VE/mg extract), 20.97 micrograms orientin equivalents per milligram of extract (µg OE/mg extract), and 100.09 µg/mg, respectively. Additionally, there were significant correlations between the polyphenolic compounds, flavonoid C-glycosides, and antioxidant activity for all parameters based on the Pearson correlation analysis. This indicates that OPLs have potential as a natural source of phenolic compounds, especially flavonoid C-glycosides, with beneficial free radical scavenging activity that can be incorporated in food and pharmaceutical products.