Do I need FDA approval to sell an oily scalp treatment in the US?

If your oily scalp treatment makes a dandruff control claim using an OTC monograph active like zinc pyrithione at 1–2%, it is regulated as an OTC drug under FDA rules, not a cosmetic. This requires following the OTC monograph framework and appropriate labeling. A leave-on serum making only cosmetic claims (moisturized, refreshed scalp) is regulated as a cosmetic under the Modernization of Cosmetics Regulation Act (MoCRA). Your cosmetic chemist should clarify the regulatory pathway based on your intended claims.