Can you take oil of oregano with your other multivitamins and nystatin oral mouthwash?

Not really necessary: I presume that since you are on Nystatin you have thrush. Don't know what the oregano oil would do to the situation. But with thrush you don't want it to stay around any longer than possible. That said..use the Nystatin as direct, don't do any additives in your life and let the condition be cured. The vitamins won't hurt if you're taking the recommended dose. stay well.