Edit 2: Disregard a lot of this... I thought OP was taking hydrocodone before oxymorphone and not oxycodone. Still lots of relevant info I think.
Opana is a very potent opiate and causes more sedation than codeine formulations IMO. I kind of feel what your saying and it is weird to switch you from a weaker less sedating opiate to a stronger one when you do not seem to be abusing, as you are using less than recommended it sounds. It may be due to taking the Ibuprofen as it is not supposed to be mixed with acetaminophen, for example whenever they gave it to me in the hospital it would never be together and very limited.
The weird thing about opana is the bioavalibility is not only low at 10%, but very random as if you eat a fatty meal it will raise to 30%. Drinking is just a nono with it as it could either reduce the effectiveness of it or increase it almost 3 folds.... It just becomes risky to do that. Maybe suggest oxycodone ER with IR for breakthrough if needed to replace the hydrocodone as it is similar in effect and chemical structure, but stronger than the hydrocodone.
In all honesty I feel it is weird your doctor chose oxymorphone over oxycodone when the latter is more closely related to hydrocodone in structure and effect where the first is a completely different effect while it may be more useful in some situations. Personally I use oxymorphone at night and hydrocodone during the day if needed.
I think the best route of action would be to try the oxymorphone and see if it works. If not bring up your concern with your doctor and ask why he changed the medication when it was working so well you did not need to use as much. If I am right about the possible negative combo of vicodin and ibuprofen than I would bring up replacing it with oxycodone either ER, ER+IR, or just the IR as they are all pure formulations and would not have that dangerous interaction plus it would function like hydrocodone, but just require a lower dose.
Just my thoughts on this. Definitely do some research into the different compounds and figure out what the best route of action is on your own and with your doctor.
Edit: This article below may help.
http://www.painphysicianjournal.com/2008/april/2008;11;S133-S153.pdf