Fine-tuning the superparamagnetic (SPM) properties of iron oxide nanoparticles (NPs) through precise control over size, shape, and assembly into superclusters is essential for advanced biomedical and electronic applications. We first analyzed the size-dependent magnetic properties of FeO@FeO core/shell NPs in both spherical and cubic shapes prepared via the thermal decomposition of iron(III) oleate. The detailed analyses of structure, composition, and crystallinity confirmed the presence of both FeO and FeO phases and the formation of the core/shell structure, with an increasing FeO/FeO phase ratio correlated with larger particle size. Overall, the SPM properties of these core/shell NPs were maintained, although saturation magnetization and varied with size, shape, and FeO/FeO ratio. Notably, iron oxide nanocubes exhibited enhanced saturation magnetization compared to their spherical counterparts. Next, we introduced a unique strategy to enhance and fine-tune the SPM properties of FeO@FeO NPs by assembling them into supercluster particles to promote interparticle interaction. By controlling the size and shape of the primary nanocrystals, we demonstrated the creation of SPM superclusters of consistent sizes, including the 150 and 240 nm superclusters reported here, which exhibit different SPM behaviors. Our research presents a synthetic strategy for optimizing the SPM properties of iron oxide NPs and their superclusters across a wide range of magnetically driven applications, especially useful for biomedical technologies.