Welcome Donna!
There are three very important aspects of successfully treating diabetes in cats, and if all three recommendations are followed, the majority of cats go into remission for their diabetes (which means they no longer need insulin).
First, is food. There is no Hills food that is acceptable for a diabetic cat, even the prescription ones they claim are for diabetics. There is also no dry food that is ok to feed a diabetic. Diabetic cats need a low carb (less than 10%), canned diet at all times. The good news is that there are a TON of options for food! Here's a link to the cat food nutrition charts:
http://www.felinediabetes.com/diabetic-cat-diets.htm. Just pick something that is under 10% in the "carb" column. Many people here feed the Fancy Feast "Classics" because they are easy to find and most cats love them. Just try and limit fish to once or twice a week--there's a high mercury content and cats can get addicted to it.
This is very important, though--do not remove the dry food until you are home testing and lowering the insulin dose. Removing dry food can cause Blood Glucose levels to immediately drop by 100-300 points, and some cats even go into spontaneous remission.
The second important point is insulin. Unfortunately, your vet has you using a dangerous and ineffective insulin for cats. Humulin-N (NPH) is a good insulin for dogs, but not for cats because cats' metabolisms are twice as fast as people or dogs, so the Humulin only lasts a short time. Cats do extremely well on long acting human insulins like Lantus or Levemir because they last a full 12 hours per shot. Here is a link to a document for you to print out and give to your vet:
http://www.uq.edu.au/ccah/docs/diabetesinfo/link2.pdf. Note where it states "Lente is the 3rd insulin of choice and
NPH the 4th of choice insulin for control of diabetes mellitus in er cats,
behind glargine or detemir (1st choice) and PZI (2nd choice), Lente and NPH result in lower remission rates compared to long acting insulins." I would urge you to get your vet to change the insulin immediately. Not only is Humulin-N rather ineffective (which means Oreo's diabetes will most likely worsen), Vets tend to overdose it because it is ineffective which often leads to dangerous hypoglycemic incidents. It's really in both your and Oreo's best interest both health-wise and financially (even though it seems less expensive than the other insulins on the surface, the Humulin is a waste of money which usually leads to more expensive vet bills) to ditch the Humulin asap and get a better insulin.
Finally, when dose adjustments of a good insulin like Lantus, Levemir, or Prozinc are made via daily home testing, newly diagnosed cats have an high remission rate. Home testing seems hard at first, but after a week or two (once the cat is used to it), it's incredibly easy. Vet testing is inaccurate because stress will raise a cat's blood sugar levels at the vet, and this too often leads to chronic overdosing. Also, home testing will save you a ton of money because it is far cheaper to do it yourself than get curves done at the vet. The Relion meters have inexpensive strips and are good quality--you can get them at any Walmart if you're in United States. If you need any help or tips learning to home test, ask, because we have a ton!
I know that what I'm saying is probably contradicting what your vet is telling you, but understand that many vets are not current in feline diabetes treatment. Many of these guidelines recently came out in 2009, and many vets just haven't seen them yet. I would strongly recommend you print the documents I'm providing below and bring to your vet to bring him/her up to date. If your vet is stubborn and refuses to change treatment despite the information you're providing, you may need to change vets.
American Animal Hospital Association diabetes guidelines, which recommend a low carb, canned diet, home testing over vet testing, and a long acting insulin:
http://www.aahanet.org/PublicDocuments/AAHADiabetesGuidelines.pdf
Article which shows the safety and effectiveness of Lantus and Levemir along with daily home testing and a low carb, canned diet: