What OTC nausea medication to use for migraines or general nausea?

Benadryl (diphenhydramine) Antihistamines can help with nausea. Benadryl (diphenhydramine) is a good option to try. You should discuss with your doc.
Nausea. Resulting from migraine often requires treatment of the underlying migraine with migraine specific otc or prescription drugs. Effective anti nausea meds are by prescription.