Understanding the Concept:
An odd electron molecule contains an odd total number of electrons and is usually paramagnetic.
Step 1: Count total electrons in each molecule.
• CO: \(6 + 8 = 14\) (even)
• SO$_2$: \(16 + 2\times8 = 32\) (even)
• CO$_2$: \(6 + 2\times8 = 22\) (even)
• NO: \(7 + 8 = 15\) (odd)
• OF$_2$: \(8 + 2\times9 = 26\) (even)
Step 2: Identify odd electron molecule.
Only NO has an odd number of electrons.
Step 3: Final Answer.
\[
\boxed{\text{NO}}
\]