The oxidation number of Mn in the product of alkaline oxidative fusion of MnO2 is:

2MnO2+4KOH+O2→2K2MnO4+2H2O
Oxidation state of Mn in K2MnO4=2+x+(-8)=0→x=+6
Oxidation state of Mn=+6 in K2MnO4 (Potassium manganate)