Gate-all-around integrated circuit structures having oxide sub-fins, and methods of fabricating gate-all-around integrated circuit structures having oxide sub-fins, are described. For example, an integrated circuit structure includes an oxide sub-fin structure having a top and sidewalls. An oxidation catalyst layer is on the top and sidewalls of the oxide sub-fin structure. A vertical arrangement of nanowires is above the oxide sub-fin structure. A gate stack is surrounding the vertical arrangement of nanowires and on at least the portion of the oxidation catalyst layer on the top of the oxide sub-fin structure.