You can go for Mapping by SEM or FESEM and then calculate amount of oxygen (per unit area) adsorbed ( if your material surface already does not have oxygen) by some image analysis software like Image J or etc.
ESCA (XPS) or Auger can be used to look at the oxidation state of elements on the top 5 - 10 atomic layers. In air all species will be oxidized. A chemisorption experiment (cf BET) with a preparative reduction stage (to remove oxide) followed by oxygen chemisorption can provide the specific surface area amenable to oxygen.