THOUGH FLOURINE IS MORE ELECTRONEGATIVE THAN OXYGEN BUT OXYGEN HAS MORE ABILITY TO STABLISE THE HIGHER OXIDATION STATE. WHY ??
F is more electronegative than O.The ability of fluorine and Oxygen to stabilize high oxidation states is due to their small size and tendency to attract electrons. F stabilizes higher oxidation states by all single bonds as IF7. 7 Fluorine bonded to 1 atom Iodine.But in case of Oxygen it can form double bonds so higher oxidation states are stabilized by bonding less no. of oxygen to central atom.Ex. OsO4 , 4 oxygen atoms stabilizes the oxidation state of +8. That is why Oxygen is more favorable.