Hi, Belle. Welcome to DC. The cookie you remember sounds interesting. The way you describe it, it sounds as though your grandmother is not with you anymore. If that's the case, is there another family member who might have the recipe? If not that, then, have you tried to Google the recipe or similar ones?
As for the oats, sure, you can probably process them to make them finer. It's worth a try. Do it and see what happens. You really have nothing to lose.
Hang in there. More answers will come. Hope you come to love DC like I do. We're glad you've found us.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!