Many online dictionaries don't even have
oaty, let alone
oatier. However it is a validly constructed derivation of
oat(s), and its meaning is obvious.
It doesn't get a lot of use in published writings. One example is from a 2016 article in
Wired magazine, "What's the best biscuit for dunking? We use robots and physics to settle the debate":
“The Rich Tea impressed us all,” says Barnes. “Its compact structure means it can hold its shape a bit easier than the looser, oatier biscuits.”
I'll add
oatier as a rare word and, for consistency,
oatiest as well.
