Overall, 0. The H atoms have a ON of +1 each (or plus two altogether) and O is -2.
This comes straight from the oxidation number (or ON cuz I’m lazy) rules. The particular rules that I drew from are bolded;
The sum of all by itself equals 0 on a neutrally charged molecule.##H_2O## has no positive or negative charge, thus its overall ON must be zero, 0.
Hydrogen has an ON of +1 when with non-metals.In the water molecule, H is with O. As oxygen is not a metal, each hydrogen molecule must have an ON of -1. As there are two H atoms, the total ON for H is ##-1*2=-2##.
Oxygen has an ON of -2 As the rule states, in this case O has an ON of -2.
The O and H ONs can be added together to get an overall ON. ##-1(2) + -2 = 0##. As can be seen, this works out with the first rule!
I found this a good overall ON explanation.Tyler DeWitt – How To Calculate Oxidation Numbers