Step 1: Identify the structure.
Oleic acid is an 18-carbon monounsaturated fatty acid, denoted as C$_{18}$H$_{34}$O$_2$, with one cis double bond at the 9th carbon.
Step 2: Determine saturation.
Because of the presence of a C=C double bond, it is unsaturated.
Saturated fatty acids contain no double bonds between carbons.
Step 3: Conclusion.
Hence, oleic acid is an unsaturated fatty acid.