We established the dual role played by oleylamine (OAm) during the synthesis of CdSe nanocrystals (NCs) by the hot injection method. Earlier reports suggest its role either as nucleating or as passivating agent in controlling the growth of CdSe NCs. Remarkably, by exploring four different synthesis routes, in which the reactant addition, timing, and concentration were varied, we found that both these two phenomena coexist and control synthesis. While examining if there is any effect of concentration of OAm on this synthesis, we found that at lower contents of OAm (<0.5 mmol) it prominently acts as an agent acting on nucleation, increasing the number of nuclei, reducing the nuclei (initial monomer units) size, and thereby increasing the NCs concentration resulting in a small NCs size, down to 2.7 nm. Whereas at higher contents (>1.0 mmol), it served more as a passivating agent by deterring both nucleation and growth processes, so generating fewer NCs with larger size up to 3.6 nm. Thus, adjusting the influence between nucleation and passivation, we can better control the final NCs size and so tune their size-dependent optical properties.