Well, fleas is one concern. However, food and enviornmental allergies (atopy) are just as common. I also can't ruleout reactions to any products you have used (i.e. even the oatmeal shampoo if all the residue is not washed off can irritate the skin).
So, here's some info about both allergies and various treatment options. Many require your veterinarian's care, while some reacommendations are over-the-counter.
So, to rule-out food allergies you need to do a diet trial.
Summary: The primary protein in most dog foods is lamb, chicken, or beef. The primary carbohydrate is rice or corn. So, we need to change both. The most popular alternative diets are Venison and Potato, Duck and Potato, or Fish and Potato. It is available in prescription formulas (i.e. Science Diet D/D) or you can find some similar foods at the big pet stores like PetSmart or PetCo. Just make sure that when they say Venison is the "main ingredient" they don't sneak in other stuff like lamb meal or fish meal. Those would be poor choices. A non-prescription option would include Dick Van Patten's brand called Natural Balance. Keep in mind, holistic or all-natural foods are not necessarily any better when trying to remove a food allergy. Chicken is chicken, and rice is rice. When you perform a diet trial, you must stick with the food for 2-3 months to see if there is any benefit. No other treats or human food can be given during this time period.
Also, definitely read about ENVIRONMENTAL ALLERGIES (atopy). Some pets, suffer from both environmental and food allergies:
If you want to actual diagnose what in the environment may be causing the allergy, you can do an allergy test. Now, you don’t have to shave half the body and do the old grid test on the skin. There is now a rather good blood test that looks for allergies to weeds, trees, grasses!, molds, even house dust mites. You can read more about it here:
Now, in addition to allergies, there are also some other common causes of skin problems or secondary infections that can occur:
Mites (demodex or scabies). Demodex is not contagious, however, scapies or sarcoptic mange is zoonotic. So, keep in mind, when ever someone mentions mange, it is important to distinguish between which mite. To diagnose if there are mites, your veterinarian can perform an easy skin scraping test in the hospital. It is not always 100% accurate, but a good start. It can be difficult sometimes to scrape out the mites.
Skin scraping: http://www.petplace.com/dogs/skin-scraping/page1.aspx
Secondary yeast or bacterial infections (very common)
Frequently, the skin will be inflamed, have an odor, and/or oily component.
TREATMENT OPTIONS (A small representative list)
Steroids. Veterinarians will either give an injection that can last from hours to weeks, or send home oral steroids like prednisone or dexamethasone.
Antihistamines. A prescription of hydroxyzine is common. Sometimes, over-the-counter Benadryl will help.
Benadryl can be given at a dose of 1mg per pound of body weight. Keep in mind, Benadryl tablets and liquids come in different sizes. So, an approximately 25 pound dog can get a full 25mg tablet or a half of a 50mg tablet. I usually avoid the liquid Benadryl in larger dogs (you would have to give too much of it). You can give Benadryl every 8-12 hours.
Essential fatty acid supplements. Definitely, if you plan to give an antihistamine long-term,
combine it with an EFA supplement. I prefer the veterinary products dosed for pets. Your vet
should carry a product on their shelf, or over-the-internet you can look for Aller G3, 3V caps,
EFA-z, or Derm caps.
Some vets use a product called Temaril-P. It’s an antihistamine with a low-dose steroid built into
the same tablet.
Atopica. Neat stuff. Can take a few weeks to be of benefit. Not the most cost-effective, but can
be incredible for allergies when the above does not work. Read the link below for more
Your vet may also carry some special sprays or shampoos for the skin. Some contain topical
steroids with antibiotics.
One of my favorite product lines which you can research on the internet are the products by Douxo. They also have an excellent shampoo called "Douxo Calm shampoo"
Lastly, oral antibiotics or antiyeast medication, if indicated.
That's about the most I can say at this point. Well worth the veterinary checkup to get the itching under control.