Step 1: Know what formal charge means.
Formal charge tells us if an atom in a molecule seems to have gained or lost electrons compared with its free state. We need it for the oxygen in carbon monoxide, CO.
Step 2: Write the formula.
\[ \text{Formal charge} = V - L - \frac{1}{2}S \]
Here $V$ is the valence electrons of the free atom, $L$ is the lone (non-bonding) electrons on it, and $S$ is the shared bonding electrons around it.
Step 3: Build the CO structure.
Carbon brings 4 and oxygen brings 6 valence electrons, so 10 in total. To give each atom an octet, they form a triple bond, written $:\text{C} \equiv \text{O}:$, and each atom keeps one lone pair.
Step 4: Read off the numbers for oxygen.
For oxygen: $V = 6$. It has one lone pair, so $L = 2$. The triple bond means 6 shared electrons, so $S = 6$.
Step 5: Put the numbers in.
\[ \text{Formal charge} = 6 - 2 - \frac{1}{2}(6) = 6 - 2 - 3 = +1 \]
Step 6: Quick check with carbon.
Carbon gives $4 - 2 - 3 = -1$. The two charges add to zero, which fits a neutral molecule. So oxygen carries $+1$.
\[ \boxed{+1} \]