Speaking for the US, yes indeed. Loperamide is technically an opioid and available OTC, though that's rarely a sought after compound unless one is in dire straits.
Codeine is available with a pharmacist's approval alone in my state. So while it may not technically OTC by that definition (or is, I don't know how OTC is defined), it can be had without a prescription by visiting a participating pharmacy and simply requesting codeine+guaifenesin. Done it a few times myself.
Codeine + cough syrup is so popular in Texas that it spawned an entire subculture dedicated to its use. See DJ Screw in Houston (https://en.m.wikipedia.org/wiki/DJ_Screw).
Lest you think this is no longer the case - “lean”, as it is known by Houstonites (due to your tendency to lean on things while under the influence) was a major plot point of a recent mostly autobiographical Netflix series centered around Houston that was released late last year: https://www.keranews.org/arts-culture/2022-08-25/houston-ooz...
Vice released an excellent short piece on Houstonian screw & slab culture years back. Might still be on youtube! It was an excellent piece at the time, but I haven't seen it in quite a while.
For clarity, loperamide, sold under the brand name Imodium, is an anti diarrhea medication. It is an opioid; however, it does not readily cross the blood-brain barrier. It only acts on the opioid receptors in the large intestine and slows the movement of food. In very high concentrations it may get into the brain.
"A pharmacist's approval" is a prescription, just one that happens to be written by the same person who fills it. So no, not OTC, which specifically refers to medications you can get without a prescription.
I suspect this is location specific though. In the UK my migraine medication was available over the counter, I still had to have a discussion with the pharmacist every time I bought it. Conversely when ever I've needed an actual prescription medication, in an emergency or whatever, that has always had to go through my doctor to get the prescription sorted.
My go-to pain-relief used to be a product called Codis, which was soluble aspirin+codeine. It used to be available OTC, until about 8 years ago, then it disappeared. You can still buy paracetamol+codeine OTC ("Paracodol?). My understanding is that aspirin causes vasodilation, which causes more rapid absorption of codeine. Supposedly that's why Codis worked better than Paracodol.
I don't know why they took it off the shelves. There was no announcement; it just disappeared.
Codis was effective for both migraines and period pains, as reported to me (I'm not a woman, and I don't suffer from migraines).
This was migraleve it didn't have any painkilling element in the first tablet (it was 2 parts) not much point anyway as by that point I was throwing up.
Well, sure, countries will have their own laws over this or anything. Ours are defined at the federal level and thus apply the same everywhere in the country; since GGP and I are both USian, that's the basis on which I replied. (I don't know enough about any other country's such laws to speak to them!)
Codeine is available with a pharmacist's approval alone in my state. So while it may not technically OTC by that definition (or is, I don't know how OTC is defined), it can be had without a prescription by visiting a participating pharmacy and simply requesting codeine+guaifenesin. Done it a few times myself.