Diamorphine is given IV/IM/SC - not orally - which generally means that its use is confined to a short period in the first few hours/days of a hospital admission until the patient can be switched to oral medication. Noone is sent home with diamorphine unless they are end stage palliative and they are on a home syringe driver to get it.

OTOH oxycodone is almost always orally administered which means patients stay on it for longer in hospital and are commonly discharged with a prescription for more.

The doctors I work with who have done fellowships in USA state that oxycodone is (?was) given out like lollywater compared to hospitals here, where it is only prescribed if the patient has a REALLY good reason they can't have oral morphine.