The Austral Opossum is one of the most unique vertebrates in Patagonian forests. It is not only endemic to this habitat; it also plays a key role acting as a seed disperser, nest predator, and is prey to species of conservation concern. However, specific information about its nesting behaviour is scattered and rudimentary at best. Here, we: (i) use personal observations along with published data to group information about nest site choice and (ii) investigate how flexible this behaviour is, examining the relative frequency of both cavity (natural and artificial) and non-cavity nests. We found that Opossums placed their nests inside cavities 50% of the time in old-growth forests but only 25% of the time inside cavities in second-growth forests. We suggest that our naturalist records, along with the previous published literature, might reflect a flexible response of the species to cavity availability. In forests suffering continuous degradation, such as Patagonian ecosystems, this flexible behaviour may increase the likelihood of population persistence in the future. This study leads to the following further questions: is Austral Opossum nesting behaviour phenotypic flexibility or plasticity? Are cavities a limiting resource for this species? Would it be more efficient for a conservation program to install nest boxes or to take actions to encourage the understory vegetation where they can nest?