Yea, I totally agree with psycho_logic regarding the dose. Definitely don't fuck with the Methadone dose at all if you're looking to get the most out of your pain medication. 90mg of Oxycodone per day is a relatively high dose, especially in the current climate of limited prescribing. I think what the Methadone will do that will benefit you the most, is that it will smooth out the edges of your pain control.
Oxycodone, compared to Methadone, is a short-acting Opioid. It's only going to last 4-6 hours, with 4 hours of serious analgesia to be expected in most people. Taken orally, you'll experience onset of effects within a half hour given normal stomach content. Methadone is much different. It is a long-acting Opioid. To compare the two, Methadone is going to start working at about an hour when taken orally and is going to produce analgesia for at least 5-6 hours and for many 8 hours. Now this only applies to taking a single dose. Methadone will build up to what is called a "steady state", that is, a stable level, at about day 5 of a given dose change.
Once this steady state is reached, you will have Methadone working in a behind the scenes kind of role, providing pain relief around the clock at a relatively low level. Meanwhile, you will continue taking your Oxycodone as usual and this is going to provide you with the same level of pain relief as it is now, only with the added benefit of the Methadone cushion making you feel a little more stable inter-dose.
Does that make sense?