I don't recall where I read it (perhaps BYO many years ago) but I have added oats to the boil.
Many years ago. At the time I was brewing all extract, and this was about the only way to get oats into the beer.
These days, I mash them with my base grains.
I can't recall what impact they had on flavor, but I'm still brewing the same base recipe so it must have been positive.