TL;DR: A commentary on Maurice's answer.
We also know that the oxidation number of a monatomic ion is the same as the charge of the ion, does the same apply for a polyatomic ion?
This has one major gap in understanding. An oxidation state is defined for an atom, not a molecule. Look at the definition stated in the gold book,
In English is largely synonymous with oxidation state, and may be preferred when the value represents a mere parameter or number rather than being related to chemical systematics or a state of the atom in a compound.
Gives the degree of oxidation of an atom in terms of counting electrons. The higher the oxidation state (OS) of a given atom, the greater is its degree of oxidation. Definition: OS of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds.
Both of these definitions have one thing in common. It talks about the state of an atom not a molecule. Hence, there is no such thing as an overall oxidation number.
- The IUPAC Compendium of Chemical Terminology. ** 2019. DOI: 10.1351/goldbook.