Zinc containing ternary nitrides, in particular ZnSnN2 and ZnGeN2, have great potential as earth-abundant and low toxicity light-absorbing materials. The incorporation of oxygen in this system - may it be intentional or unintentional - affects the crystal structure of the materials as well as their optical band gaps. Herein, we explore the origins of structural changes between the wurtzite type and its hettotype, the β-NaFeO2 type, and highlight the effect of oxygen. Furthermore, we study the electronic structure and bonding in order to understand the reason for the narrower band gap of zinc germanium oxide nitrides as opposed to pure zinc germanium nitride.