Why would my mother's doctor prefer oxycodone over tramadol?

Talk to your pharmacist also. They are an excellent resource about drugs.

What kind of pain does your mother suffer from? From a brief review of comparison charts online, it seems to me that oxycodone is a more powerful pain med. Does your mother have a condition in which one can reliably predict that her pain will worsen?

Tramadol isn't getting such a good press at the moment; though I appreciate that oxycodone has its drawbacks too.

Keep careful records of how she gets on and if you do find she's getting new and/or worse problems, or less effective pain relief, you can always ask the doctor to reconsider.

You can download up-to-date patient information leaflets for both drugs, make diplomatic but objective summaries of both, and point out to your mother that this is a case of swings and roundabouts. I also would not be satisfied with the nurse's reply: I think it would be reasonable politely to insist on an answer that explained the benefit of this change specifically to your mother. You'll probably need to get that from the prescribing doctor, though - again, a perfectly reasonable request to make.