Ten years after the first report on ferroelectricity in HfO2, researchers are still occupied unraveling the different causes behind this phenomenon. Among them, oxygen related defects seem to play a major role, affecting both crystalline phase formation and performance of HfO2-based devices. This review surveys the available literature and provides a broad picture on the topic, starting with an overview of existing oxygen-related defects, assessing the extensive calculations and experimental reports on phase stabilization in both undoped and doped HfO2 and concluding with a discussion of device reliability involving oxygen vacancies, first in more classical HfO2 applications such as MOSFET high-k metal gate and resistive switching devices and later in the three major groups of ferroelectric non-volatile memory devices.