As for the oatmeal bath, here is a link for a homemade oatmeal bath:
Homemade Oatmeal Bath - How to Make Your Own Oatmeal Bath but if I was going to go that route, I would just buy the packets of Aveeno Oatmeal Bath. I have used those for my kids and they do take away the itchies.
This is probably silly to ask because I know you take such good care of Noonie, but are you sure you are rinsing well enough? We have softer water here, and it is very difficult to make sure they are rinsed well enough. I just keep doing it til I think I am done, and then rinse a couple more times. I have had mine itch a little more than usual, and the first thing I do is give them a bath, in case they got into something that is an irritant to them, and to make sure I get them rinsed really well.
Allergies are difficult to pinpoint, could be anything from carpeting, shampoo, food, to outside environmental things. Wish you good luck in narrowing it down. I know I worry when they itch too. You just know that is not comfortable for them.