The colloidal synthesis of high entropy oxide (HEO) nanocrystals requires navigating and balancing competing reaction pathways, which are often unknown. There is also a limited understanding of HEO nanocrystal formation and growth pathways, which hinders morphological control. Here, we report on the colloidal synthesis of rocksalt-type (FeCoMnMgZn)O nanocrystals with different morphologies. Reaction pathway studies show a Fe-rich spinel-type intermediate and indicate that competing chemical reactivities dictate the final compositions and morphologies. Atomic resolution imaging analysis of concave cubic and dendritic (FeCoMnMgZn)O nanocrystals indicate a 1.73% lattice expansion relative to bulk FeO. The (FeCoMnMgZn)O nanocrystals are active electrocatalysts for the oxygen evolution reaction in alkaline media.