There is nothing OTC that makes opiates more euphoric, sorry. Potentiators will either help analgesia/sedation (antihistamines, anticholinergics) or make the drug last longer/be converted to active metabolites more effectively (enzyme inhibitors/inducers respectively) but they won't actually make your drugs feel any 'better'. The best general-purpose OTC potentiators are generally sedative antihistamines like diphenhydramine (Benadryl), dimenhydrinate (Gravol), promethazine (Phenergan), or doxylamine (Unisom) taken as recommended for allergies (usually 25mg diphenhydramine equivalent).
If you want to get into mixing drugs though you can try mixing either marijuana, dissociatives, or other depressants with your opioid. I reccomend only adding one drug at a time and only using drugs you are experienced with. (Don't do a cocktail of Vicodin and Soma if you've never done Soma.) But be warned that mixing depressants like Soma, benzos, barbiturates, and alcohol with opioids can cause an increased risk of unconciousness, vomiting, and death.
Another option is trying a different opiate, or simply taking more.