You may want to check w/ your pharmacy, the MD, or your charge nurse, or the other nurses, to see what the hospital rule is, or the "floor culture" is, to make sure you're not flying rogue unknowingly - but, personally
1) if a Pt has been having trouble w/ pain, I use what I have to steady out his pain. So if pain is 8/10, I'll go w/ dilaudid, and then in an hour, when the dilaudid has brought the pain to a more manageable level, I may give the oxycodone or percocet to keep the pain from shooting back up again. This can lower the number of times that he may need IV dilaudid.
**Of course, you want to make sure to assess the Pt's LOC, how well he's breathing, possibly O2 sat, etc., to make sure it's safe to give another pain med after the first one** although if it's been an hour, the dilaudid has peaked
2) sometimes I'll give an oral med (oxycodone or percocet) when the pain is moderate to try to keep it from going high to the point that the Pt needs IV med. But if the oral med doesn't work, and the pain is climbing, then I may follow with IV narcotic - again, after checking LOC, responsiveness, etc.
**Caution: the oral med takes longer to kick in and also longer to get out of the Pt's system, so if you follow it w/ an IV med (as opposed to giving an IV med and then thirty minutes/sixty minutes later an oral med), you do risk the IV med hitting the Pt at the same time that the oral med is becoming effective. So I warn the Pt of this, make sure that they don't get up w/out calling for help after taking narcotics, and monitor him. I take baby steps w/ giving 2 meds in succession if I haven't had the Pt before and don't know his ability to tolerate narcotics and/or if other shifts haven't been giving meds this way w/ the Pt tolerating it. That is to say, go w/ a smaller dose at first and monitor the Pt! Explaining what you're doing and the different time frames for effectiveness/peak/duration of IV vs PO meds also helps the Pt understand, and participate in the plan of care, and to help you make things safe.
And don't forget that you can contact the MD - either to get an order (at least a "Yeah, that's okay" which I can chart) that meds can be given successively (this covers your butt), or to let him know pain meds aren't working, so he can dose pain meds appropriately.