Can I use human shampoo on my dog?

You can, but when there are thousands of options for dogs, why would you? Dogs PH is different, however every dogs PH is different from one another as is the shampoo. Oatmeal is not a good option for dogs, they shouldn't have any grains or grain products.

It can be very tricky determining if a human product is appropriate for a canine. One vet or expert may tell you one thing and another contradict their opinion. Also, google searches to determine if each ingredient is safe can lead to contradicting answers. It is always safest to use a trusted dog shampoo, preferably natural. While many human products may be perfectly fine, I don't take chances.

no, it will dry out their skin. There PH balance is different than ours. I would recommend that you use a doggie shampoo. If you apply topical flea and heartworm medication than you want to use a shampoo that does not strip away the medication. Using like an oatmeal shampoo would be good. Happy Bathings!