The flavor profile of oatmeal raisin cookie might be better suited for a brown ale or porter. More chocolately, less dry/roasty.
If you're mashing, I would use toasted oats to add some oatmeal cookie flavors.
For specialty grains, a bit of C60 will give some dark sugar flavor and sweetness.
A bit of molasses at the end of the boil will add to that rummy dark brown sugar note.
Adding cinnamon at knockout or in secondary will resonate the oatmeal cookie flavor. Use cinnamon sticks. Be careful about adding to secondary - a little goes a long way, and this is only a background note!
I like Denny's method of using raisins, and I'll probably try it next time! I have also pureed raisins with hot wort and added them at knockout (this is how DFH adds raisins to Raison D'Etre)