FeO has a cubic inverse spinel group structure which consists of a cubic close packed array of oxide ions where all of the Fe ions occupy half of the octahedral sites and the Fe are split evenly across the remaining octahedral sites and the tetrahedral sites.
Both FeO and γ-FeO have a similar cubic close packed array of oxide ions and this accounts for the ready interchangeability between the three compounds on oxidation and reduction as these reactions entail a relatively small change to the overall structure. FeO samples can be non-stoichiometric.
The ferrimagnetism of FeO arises because the electron spins of the Fe and Fe ions in the octahedral sites are coupled and the spins of the Fe ions in the tetrahedral sites are coupled but anti-parallel to the former. The net effect is that the magnetic contributions of both sets are not balanced and there is a permanent magnetism.
In the molten state, experimentally constrained models show that the iron ions are coordinated to 5 oxygen ions on average.  There is a distribution of coordination sites in the liquid state, with the majority of both Fe and Fe being 5-coordinated to oxygen and minority populations of both 4- and 6-fold coordinated iron.