Oil content varies. I've seen anywhere from 3% (Briess) to 7% (store brand).
I wondered if some metabolisation and reduction of oils occurs during malting of oats. Didn't find anything.
My suggestion to Luke is to repeat the recipe and sub barley or wheat for the oats. If the slick still forms, report back. If that's a cure, you can blame the oats, but I'm still skeptical. In that case try oats or oat malt from a different source.