The rock salt-to-spinel structural transformation that occurs in anodically electrodeposited Mn-Co-O nanocrystals involves a rearrangement of Mn/Co cations from octahedral interstices to tetrahedral interstices. The cation ordering process leads to distinct magnetic properties. Curie temperature (T-C) and blocking temperature (T-B) increase dramatically with annealing temperature (200-400 degrees C), while the corresponding change in particle size for the oxide nanocrystals is rather small. A strong correlation between the magnetic properties and the cation ordering degree in annealed Mn-Co-O nanocrystals was established. These unique magnetic properties can be attributed to the magnetic moment changes induced by Mn/Co cation ordering from octahedral interstices to tetrahedral interstices in the annealed Mn-Co oxide nanocrystals.