The two substances don't really have an interaction. In general though, taking oxy and kratom at the same time or within the same day on a regular basis will raise your opiate tolerance quite a bit rather quickly.
You can take them both at once or separately--doesn't really matter too much. It's not like you won't feel the oxy if you've taken kratom long before or anything like that. I like taking them separately. PS: kratom doesn't cause respiratory depression even in higher doses. So if you're asking this in terms of safety precautions, you don't have to worry. I've taken a shit load of kratom on benzos before. There's no dangerous interactions with kratom. Kratom does however raise your tolerance to opiates in general. For example, I take it every day and when I go back to oxy I have to take so much more oxy to feel the effects as opposed to when I was only taking pills daily. Oxy and kratom when taken together cancels out the high and improves it in other ways. Too hard to describe in text. But the two together destroys your physical pain, that's a given.