Biomass derived from agricultural waste is a promising source of renewable energy. When used in low-emission combustion technologies such as chemical looping combustion (CLC), it has the potential to achieve net negative CO2 emissions. In CLC, the fuel is isolated from atmospheric air, resulting in flue gases that comprise mainly CO2 and H2O. Since the fumes are not diluted by atmospheric N2, low-cost CO2 capture is possible. The oxygen required for CLC is delivered entirely by an oxygen carrier (OC). Spinel-type OCs have a high oxygen-transport capacity, mechanical durability, and chemical stability. However, biomass ash is rich in alkali metals and SiO2, which adversely affect OCs by promoting cracking and agglomeration. Herein, the effect of Mg doping on the resistance of OCs to biomass ash is explored. Five MgxCu1-xFe2O3 -type spinels (x = 0-1) are evaluated for the combustion of four types of biomass with varying ash compositions: three agricultural waste products (pine wood, kenaf, and rice husk) and one dedicated energy crop (Miscanthus). Among the tested OCs, Cu0.5Mg0.5Fe2O4 demonstrates the highest reactivity and conversion rates, with a reaction rate of 2.70 wt.%/min for kenaf and 95.9 % conversion for Miscanthus. Following multiple reaction cycles, undoped and low-Mg OCs (x ≤ 0.5) exhibit cracking and structural degradation, whereas high-Mg OCs (x ≥ 0.75) retain their structural integrity, confirming the benefit of Mg doping on the durability of spinel-type OCs. This study provides insight into the design of more resilient OCs for biomass combustion, which will guide future research on CLC technologies.