Oasis Fathom is a location in Torment: Tides of Numenera.
This is one of the Fathoms unlocked after all castoffs been sucked into Labyrinth. If Callistege was in your team, she can be find here. Help her get rid of illusions and make the decision to stay human or otherwise, and she will join your team again.
Mazzof can also be found here, release him form his imaginery prison, and he will give you a cypher.
Try to capture the memory when interacting with the structure on north gives you 4 XP.