What is the oxidation number of P in PO?

Are you thinking of a phosphorus oxide (PO) or elemental metal polonium 'Po'. ????

If you are thinking of an oxide of phosphorus , then the chemical formula is P2O5, or its dimer P4O10; (PO) does NOT exist .

If you are thinking of the elemental metal polonium , then the chemical symbol is 'Po'. Note the small case 'o'.

Polonium is a highly radio-active and poisonous metal, and is kept in a very secure environment.