We present here a detailed overview on the effect of various synthesis parameters during the synthesis of iron oxide nanoparticles with different shapes, through decomposition of a preformed iron oleate complex, at high temperature. While this procedure has been previously shown to produce monodisperse magnetite spheres, the use of specific additives is demonstrated to allow for the preparation of strongly faceted iron oxide nanocrystals, with either cubic or octahedral shapes. Additionally, using squalene or octadecene as solvents was found to induce the reduction of the iron precursors and thereby lead to the formation of nanoparticles with core/shell (in the case of nanocubes) or island-like structures (in the case of octahedrons).