Two-dimensional (2D) metallic states induced by oxygen vacancies (V_Os) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of V_O-induced states at the (001) surface of anatase TiO₂, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular O₂: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface V_Os and remain localized at the surface, where they can promptly react with O₂. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with O₂ is kinetically hindered on anatase TiO₂ (001), thus making them much less sensitive to oxygen dosing.