uumpaloompa, entactogenic euphoria is very different from Opiate euphoria. I can't answer your question from personal experience, but I have used Opiates/Opioids with LSA. The physical euphoria becomes very nice, but the mental aspect of it is negative for me. Psychedelics open me up to a new world, while Opioids give me an illusion of happiness and contentment. They close me off to the world while making me feel like "everything is Ok." I didn't like that at all. I would say it's a bad idea. I know why you are wondering this, because OxyContin is extremely euphoric, and Methylone was for me as well, but I really believe mixing the two would cause an imbalance of euphoric properties. Another lighter psychedelic like 2C-C, 2C-B, a small dose of mushrooms, or some Cannabis would quite possibly enhance the euphoria a bit.
..Actually, after I posted this, I started thinking about it. I don't know how you are planning to use the OxyContin, but If you insufflated it 20 minutes after you took the Methylone, it may work well. The timing of causing them to peak at the same time may cause awesome euphoria, but then again, like I said, alot of entactogenic euphoria is psychological. Have you ever noticed that you could be in a certain mindset and setting on the same dose of a psychedelic drug one day and not trip very well or not get the strong overwelming entactogenic effects of MDMA like chemicals, then on the same dose under a different mindset and setting, you are blown away? OxyContin will get me high no matter where the hell I'm at or whether I'm happy, sad, angry, lonely, depressed, excited, etc... I have found that the best way to get the most from psychedelics and entactogens is to really "get into" the drug. Let it flow through you and don't hold back. On 150mgs of Methylone, I was blown away. On 190mgs, I hardly got much of anything. On 200mgs, I was again blown away, yet not even as much as 150mgs, though that was because I hadn't used an entactogenic drug since I had tried MDMA a few times a few years before. I would have to agree with the above reply. Opiates are better used for harsh come-downs. However, if you just had an intense entheogenic experience, Opiates will take away from that.