Author qualifications to list in medical journal
If there are guidelines they’ll be in the instructions to authors in the journal you’re targeting. 

If there aren’t guidelines, just list the degrees you have. 

Definitely don’t substitute PhD for your DPhil. It won’t be as confusing as some, and lets the reader take a view on equivalence. 

(The far more common confusion in medical journals would be the MD.  In US system medial education that’s a basic medical qualification that almost everyone has (unless they have a DO). In the British system it’s a higher degree equivalent to a PhD, and if someone lists their qualifications as eg MPH MD there’s no quick way to tell which they’ve got).