I don't always see their oil glands. The glad itself is like a bump on the lower back near the tail. Keep an eye on the feather cluster - there can be odd things that happen with feathers - I would just watch it for a few weeks, make sure nothing gets inflamed.
just remembered when i was young i had a duck with something that looked like that and it was an oil gland that had backed up. infected. if i am remembering right then a warm compress a few times a day is what is needed. hopefully someone can confirm this for you.