I'm really not sure of the exact conversion but I do know methadone is a good deal stronger than oxycodone.
I do believe that it would be possible to take methadone with oxycodone for break-through pain if your doctor prescribes it at your next appt.
One of our Community Leaders posted a medication conversion chart in her journal. It's handy for getting an idea of the equivalency of different pain meds. I only warn you that it can be kind of disappointing if you find from the chart that you're on a very low dose compared to what you were previously taking.
I hope you're able to find the dose and med combination that works for you.
Here's the link to Sandee's journal and the medication conversion tool: