Oil filler caps need to be inspected according to their type. For twist lock caps, check to see if the tang is bent or broken. If it is, the cap must be replaced.
On threaded caps, check to see if the threads are broken or damaged. If they are, replace the cap.
If the cap connects to the breather filter or the PCV hose, check these parts also, if they look clogged, they generally are and should be replaced.