Among iron oxide phases, magnetite (FeO) is often the preferred one for nanotechnological and biomedical applications because of its high saturation magnetization and low toxicity. Although there are several synthetic routes that attempt to reach magnetite nanoparticles (NPs), they are usually referred as "IONPs" (iron oxide NPs) due to the great difficulty in obtaining the monophasic and stoichiometric FeO phase. Added to this problem is the common increase of size/shape polydispersity when larger NPs ( > 20 nm) are synthesized. An unequivocal correlation between a nanomaterial and its properties can only be achieved by the production of highly homogeneous systems, which, in turn, is only possible by the continuous improvement of synthesis methods. There is no doubt that solving the compositional heterogeneity of IONPs while keeping them monodisperse remains a challenge for synthetic chemistry. Herein, we present a methodical optimization of the iron oleate decomposition method to obtain FeO single nanocrystals without any trace of secondary phases and with no need of postsynthetic treatment. The average dimension of the NPs, ranging from 20 to 40 nm, has been tailored by adjusting the total volume and the boiling point of the reaction mixture. Mössbauer spectroscopy and DC magnetometry have revealed that the NPs present a perfectly stoichiometric FeO phase. The high saturation magnetization (93 (2) A·m/kg at RT) and the extremely sharp Verwey transition (at around 120 K) shown by these NPs have no precedent. Moreover, the synthesis method has been refined to obtain NPs with octahedral morphology and suitable magnetic anisotropy, which significantly improves the magnetic hyperthemia performance. The heating power of properly PEGylated nano-octahedrons has been investigated by AC magnetometry, confirming that the NPs present negligible dipolar interactions, which leads to an outstanding magnetothermal efficiency that does not change when the NPs are dispersed in environments with high viscosity and ionic strength. Additionally, the heat production of the NPs within physiological media has been directly measured by calorimetry under clinically safe conditions, reasserting the excellent adequacy of the system for hyperthermia therapies. To the best of our knowledge, this is the first time that such bulklike magnetite NPs (with minimal size/shape polydispersity, minor agglomeration, and exceptional heating power) are chemically synthesized.