Various oleic acid salts are demonstrated to act as stabilizers for high-quality iron oxide nanocrystals, synthesized by thermal decomposition of ferric oleate. Changing the cation species in oleic acid salts allows different degrees of dissociation at high temperatures to be obtained, resulting in various stabilizer performances:  Sodium oleate as stabilizer results in monodisperse iron oxide nanocrystals of cubic shape with precisely adjustable edge lengths between 7 and 23 nm. Dibutylammonium oleate and oleic acid, in contrast, induce growth of spherical nanocrystals in the same size range. Further adjustment of the growth conditions leads to {100}-bound bipyramidal nanocrystals with a single, (111) oriented twin plane. While the nanocrystal size is related to their superparamagnetic blocking temperature, the shape influences their magnetization hysteresis properties observed at low temperatures.