Oil palm leaves (OPL), as the major component of oil palm biomass, was capitalized for the isolation of nanocellulose (NC) by unified ternary acoustic, chemical- and enzyme-assisted techniques. This study aimed to statistically optimize the extraction parameters via the Taguchi Design for sonication time, duration of enzymatic hydrolysis, the concentration of acid, and the duration of acid hydrolysis, for the highest crystallinity of the isolated NC. The optimum condition required a sonication duration of 20 min, 1 h of acid hydrolysis in 6 M of H2SO4 and 1 h of enzymatic hydrolysis that resulted in the production of NC with a maximum 68.2% crystallinity index, based on X-ray diffraction (XRD) data. Duration of sonication time and acid hydrolysis was crucial for improving the crystallinity of isolated NC. The reduced C=O and C=C peaks at 1734 cm-1 and 1536 cm-1, respectively, in the Fourier-transform Infrared (FTIR) spectra of isolated NC, confirmed the removal of appreciable amounts of hemicellulose and lignin components in the OPL fibers. FESEM micrographs revealed the needle-like structures of NC with diameters between 21‒24.8 nm. The NC isolation approach presented here offers a promising avenue to ease the use of harsh acids for biomass pretreatment.