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The formation of iron oxide nanoparticles obtained by the thermal decomposition of iron–oleate complexes via a “heating-up” process was monitored by the NMR relaxation method, which allows us to track the average diameter of iron oxide nanoparticles. The analysis of dependencies of the T 1/T 2 ratio and 1/T 2 values on time at the heating of the reaction mixture demonstrated that the nucleation and nanoparticle growth processes could proceed in two ways depending on the presence of oleic acid in the solution: continuously without separation of the nucleation stage or discretely with the separation of the nucleation and growth of nanoparticles. In the case of excess of oleic acid, the nucleation process follows the well-known LaMer model, characterized by a burst of nucleation and separation of nucleation and growth under continuous monomer supply. In the absence of oleic acid in the system, the nucleation and growth of nanoparticles are not separated and proceed continuously. The presence of a sufficient amount of oleic acid is the reason why the process of nucleation and growth of nanoparticles by heating up follows the path of “burst nucleation,” which is similar to the “hot injection” method. That is, oleic acid acts as a “fuse” for the explosive “burst nucleation.” This new approach shows that the “hot injection” method can be carried out not only mechanically (by introducing a precursor into the reaction system) but also chemically, with the help of a reagent that has been in the system from the very beginning.