Formula of Oleum is

All oxo acids of Sulphur are dibasic ;

In all oxo acids, Sulphur atom is sp³ hybridized ;

All the pi bonds are $\mathrm{d\pi }-\mathrm{p\pi }$ bonds;

Sulphur atom having 2 sigma bonds, 1 double bond and a lone pair around it has pyramidal arrangement around it ;

Sulphur atom having 2 sigma bonds, 1 double bond and a lone pair around will be tetravalent with +4 O.S (or) +3 O.S;

Sulphur atom having 2 sigma bonds, 2 double bond and no lone pairs around it has tetrahedral arrangement around it ;

Sulphur atom having 2 sigma bonds, 2 double bond and no lone pair around will be hexavalent with +6 O.S (or) +5 O.S;

Note : Do not forget to count a sigma bond between Oxygen and Hydrogen in O-H bond ;