By diffuser, do you mean nebulizer? Are they the same thing? I ask because I would not recommend nebulizing oregano oil. That would be some seriously harsh stuff. But I would recommend nebulizing colloidal silver.
A “NOW” brand diffuser from Natural Grocers. Conical about 5-6” at the base. About 6-7” tall.
I put a 3.5” PVC elbow on top of it that goes into a PVC pipe about 1.5” long that ends about 4-5” from my face while I sleep.
Sound from a very small circle of vibrating metal in the center of the bottom of the water containing part breaks the water & oils into fog.