Can methadone and oxycodone be taken together safely?

I have taken but have been told by professionals this should be avoided doing such.

They can but the methadone will block the oxycodone or any other opiate for that matter. It will be useless and a waste of your time and money to take the oxycodone with methadone. If it's a chronic pain issue then try and increase your methadone dose as it is a much, much more powerful pain killer than oxycodone. If you are dealing with acute pain then I would not use methadone at all and stick with the oxycodone.
Just a quick little story for you. When I started methadone almost 8 years ago I was still having chronic lower lumbar pain along with sciatica pain. I decided that I would take my usual 3,000 mgs of morphine thinking that it would have to help. Nothing, not a damn thing from all that morphine. It was a total waste and had I have kept taking the morphine all that would have happened was my tolerance for methadone would have increased.
One of the great things about methadone is it's extremely hard to get tolerant to it. Anyway, I got off topic,sorry, Good luck...