Lewis dot structure of carbon monoxide is $\stackrel{\circ \circ }{\mathrm{C}}\equiv \stackrel{\circ \circ }{\mathrm{O}}$. Formal charge on oxygen atom in the molecule is

Formal charge on oxygen atom:
Valence electrons = 6 (as it is in group 16 on periodic table)
Nonbonding electrons = 2
Bonding electrons = 6

So according to the formula of formal charge, you will get;

Formal charge on Oxygen = Valence electrons – Nonbonding electrons – (Bonding electrons)/2 = 6 – 2 – (6/2) = 1+