Metal oxide (MOx) semiconducting nanostructures hold the potential for playing a critical role in the development of a new platform for renewable energies, including energy conversion and storage through photovoltaic effect, solar fuels, and water splitting. Earth-abundant MOx nanostructures can be prepared through simple and scalable routes and integrated in operating devices, which enable exploitation of their outstanding optical, electronic, and catalytic properties. In this review, the latest research results in this field are illustrated, highlighting the versatility of MOx nanostructures in meeting the stringent requirements to boost the efficiency of different systems. The functional properties inherently correlate to the morphology and the crystalline habit of MOx, which in most of the cases are organized in complex heterostructures. Tailoring the assembly of heterojunctions and their electronic band structure, the catalytic surface properties and the charge transport through complex networks represent the main challenge for the transition of MOx from the research to the real-life in the field of energy conversion and storage.