Your oxycodone dose isn't really that high. I'm a chronic pain patient (non-cancer) that was prescribed 90mg of roxy every day for years(6 roxy 15s dail/180 roxy 15s every month, which wasn't enough, but, these days, I know when to shut up and be happy with 90mg of roxy per day). I got bounced off my old pain management, and my new pain doctor has me on half of that [90 roxy 15s/month], which is totally inadequate for my pain and tolerance levels. What I did with 6 roxy 15s per day was either take all 90mg at once every morning( way more effective on the pain and generally lasted most of the day), or take 4 or 5 roxys for my initial dose and have 1 or 2 for the evening. Both ways of dosing was a lot more effective than 1 pill 6 times per day. So, our doses for oxy were pretty similar. On refill days and some days here and there, I would take 150mg + as one single dose for the day. I'm also prescribed 8mg of Xanax daily, so that synergizes nicely. IME/IMO, you aren't in danger of an OD. The reason you aren't getting high on your dose as of lately is that opioid tolerance builds rapidly, and oxycodone is among the opioids that raises your tolerance faster than many opioids.
As to why the hydro feels like a "dirty high" to you, I think it has to do with the fact that hydro is more sedating (even though it's weaker than oxy), while oxy is a stimulating opioid. The nausea is probably due to all that APAP, even though you are well within the 4 grams/day safety zone. Just some thoughts and opinions from my many years of experience with large amounts of practically every opioid and opiate you can think of, including many years of IV heroin addiction, fent analogs combined with roxys, heroin, etc......
I'm sorry to hear about your cancer, I know what that's like. Be good, and I hope we all get the amounts we want of the specific opioids we like, want, and need. ~downerhead.