<p>This study reports a novel strategy for strengthening and toughening aluminum-matrix interfaces using nanosized oxide core–shell heterostructures. By leveraging optimized oxidation and tailored ball milling of high-entropy alloy (HEA) reinforcements, nanoscale oxide core–shell heterostructures were constructed in aluminum matrix composites, yielding exceptional strength–ductility synergy. Nano-MgO formed in situ within <i>α</i>-Al matrix and along <i>α</i>-Al grain boundaries under influence of plasma activation. Oxidized HEA (OHEA) reacted with Mg, leading to in situ formation of laminated Al₂O₃–Cr₂O₃–MgAl₂O₄. The gradient heterointerfaces induce a synergistic strengthening effect governed by two dominant mechanisms: Orowan strengthening derived from dislocation pinning by dispersed nano-MgO particles and heterostructure interfacial strengthening originating from the laminated oxide transition zone. The latter effectively mitigates interfacial stress concentration and facilitates efficient load transfer. Consequently, OHEA/Al composite exhibits significant mechanical improvements, with <i>σ</i>_uts, <i>σ</i>_y, and <i>ε</i>_f increased by 11.1% (306.0 to 340.1&#xa0;MPa), 15.7% (179.4 to 207.7&#xa0;MPa), and 121.0% (5.7 to 12.6%), respectively. This work provides new insights into the application of nanosized core–shell oxides for enhancing composites.</p> Graphical abstract <p></p>