OnePlus
Set a Custom Ringtone auf OnePlus
OxygenOS only lists ringtones it can find in specific storage locations. A personal mp3 file has to sit in the correct folder before it shows up as a selectable option, which is why copying a track onto the phone is not enough on its own.
Open Settings ➔ Sounds & vibration ➔ Phone ringtone and vibration. On a Dual SIM device you will also see Enable or disable Set ringtone for each SIM card (Dual SIM). Turn this on if SIM 1 and SIM 2 should use different sounds, then pick Choose SIM card 1 / 2 for the one you want to change.
Tap Ringtone from internal storage and select Choose a custom ringtone to browse the audio files stored on the device. Only files placed in a location the system audio picker can access will appear here.
If the toggle for separate SIM ringtones stays off, the phone applies one ringtone to both SIM slots. Switching it on later does not delete existing tones, it only unlocks a second selection field for the additional SIM.
A custom ringtone set through this menu applies to calls only. Notification sounds are configured separately, so a track chosen here will not automatically play for messages or app alerts.