In this contribution, we present our work on the evolution of magnetic phases in maghemite (γ-Fe₂O₃) nanoparticles upon the thermal decomposition of the surrounding oleic acid ligand layer in the temperature range between 20 and 1000 °C. Using in-situsynchrotron X-ray diffraction and total scattering, we observe a significant effect of varying amounts of oleic acid on the structural phase transitions in the iron oxide nanoparticles, transitioning from a spinel to rock salt to cubic iron. Our findings provide valuable insights into the thermal stability mechanisms of oleic acid-stabilized iron oxide nanoparticles, offering guidance for their effective utilization in diverse applications that require thermal robustness.