Poor little Oreo, I feel for him and I know how you feel also about having to give him meds that taste horrible. My husband and I always hated the idea of having to force meds on our two boy rats since it is so stressful and unpleasant for the little guys, and we did the same thing as you...tried to find anything that would taste good that we could put their meds on or something to mask the bad taste.
A couple of things that we tried that worked were Gerbers Baby Peas baby food and Gerbers Sweet Potato baby food, but the grand prize winner had to be something that our small animal vet suggested: Pina Colada syprup or mixer..you know those bottles that you buy at the grocery store or liquor store and you add rum to make pina coladas?
We found a bottle of it at our local Safeway and I would inject their meds into a piece of banana with the little syringe, then pour some pina colada syrup stuff over the banana, and coat the whole thing with dry oatmeal. The boys ate it up every time.
But, of course, they like bananas and oatmeal so if Oreo has a favorite fruit or snack that you know he loves to eat, try putting a little pina colada syrup over it, after you have put the meds on his snack. If I think of anything else that could work, I will let let you know!
But yes, it is a matter of masking that taste with something Oreo loves, and I know all rats are different. Our boys sure loved that pina colada stuff though. The vet said it is very helpful in masking the bad taste of some of those meds.