This paper presents a study on orientation domains in the vacancy-ordered titanium monoxide TiOx. Transmission electron microscopy analysis revealed the co-existence of 12 possible TiOx monoclinic domain variants, which are induced by the cubic-to-monoclinic phase transition attributed to the Ti and O vacancy ordering. These 12 monoclinic domain variants which are predicted by group theory might be randomly arranged. Furthermore, several hours of electron beam irradiation can lead to the vacancy ordering-disordering transition (i.e. monoclinic-to-cubic phase transformation) in TiOx. Our results shed light on the structural characteristics in TiOx nanostructures and thus contribute to the fabrication and design of the related nanostructures.