Actually, I remembered that there are relevant results from clinical studies that are available. Here are the side effects of OxyContin.[1] (Euphoria is considered a side effect.) The most commonly-reported side effects were constipation, nausea, and somnolence. If you look down further, you can see that both euphoria and dysphoria were reported rarely.

[1] http://www.rxlist.com/oxycontin-drug/side-effects-interactio...