Just put a soul of a dead Tame in its inventory which is located in the little pool on top of it. A minigame will start where theres a few waves of vultures trying to attack the reviving Tame, which you have to kill. After a few waves the oasi will be tamed and ready to use. Every 24hs youll be able to revive a dead tame and you can build a pretty big base on top of it so pretty worth it. Also important: dont use the most important tame, cause it will be entirely dead after taming the oasi.