OP, don't stress this, but we typically don't like to deal with threads that are dedicated solely to the recreational aspect of drug experiences, if that makes any sense. I understand that as addicts, discussing drugs, we can be a bit like ravenous dogs, but we try to keep our discourse dedicated to the safety and health of our members. We consider a person's experience with drugs to be their own personal journey. If you are ever concerned about the safety of what you're doing, we will always be here to help. As far as "extending highs", "getting higher", "what is most fun/enjoyable, it's not our thing.
This is not chastisement, I'm just trying to get you nice and indoctrinated so you can be of most use to the party and its organs. Pro tip, you're not going to find anything from OTC that will give you the meaningful "boost" that you're after with Oxycodone. Basically, if you like the feeling produced by say, an antihistamine, you can take that with your Oxycodone, but it will not produce anything like synergy. I don't want you to have to go on the same vision quest that every other Oxycodone user goes on in the hopes of extending their supply. It's a fool's errand. Can you extend the duration, effect with enzymatic inhibitors like Cimetidine (Tagamet)? Probably. Will it be to a noticeable, docementable extent? Not likely.