Spark plugs provide the very necessary function of igniting fuel in a carefully choreographed sequence. If the spark is off even a fraction of a second, that cylinder won't fire properly. In an ignition misfire, the spark plug for a given cylinder fails to ignite the fuel when it enters the combustion chamber. You would feel this as a sudden jerk of the engine while it's running.
As far as oil goes, this usually occurs because a layer of oil has coated the spark plug. As previously noted, the culprit is often a worn seal such as a gasket, grommet or O-ring. Check the spark plugs and look for an oily sheen or even soft deposits on or near each spark plug's electrode and insulator. Even if just one is fouled with oil residue, you'll want to replace the entire set.
While you're at it, check the inside and outside of the spark plug boots and clean them of any oil build-up as well, using a degreaser and a lint-free towel.