Are you confusing actually being dead with being (((brain dead)))? There are many actions controlling cell death so it's a grey area as to how long any cell or tissue is viable, but in general it's more than just oxygenation that determines the length of time an organ can be viable after death. There are many interactions that happen in cellular communication that lead to a clean cell death (apoptosis) and a dirty cell death (necrosis). It's a complicated topic that is only fairly understood which is why living organs from a not quite dead person are the standard for transplant situations. Here's a pretty good paper describing a lot of the processes of cell death and it's conditions. It's a fairly long and somewhat complex read, but it provides good insight into the nature of the various mechanisms of cell death. https://cellandbioscience.biomedcentral.com/articles/10.1186/s13578-018-0206-6