The oxidation number of nitrogen is +2.
Oxidation number of an atom is defined as the charge that an atom appears to have on forming ionic bonds with other heteroatoms. An atom having higher electronegativity is given a negative oxidation state.
Oxidation number of N in NO
The atomic number of Nitrogen is 7. The electronic configuration is
N (Z=7) = 1s2 2s2 sp3
Let the oxidation number of N be X
X + (-2) = 0
X = +2