OnePlus
Switch Between 12- and 24-Hour Clock Format auf OnePlus
OxygenOS decides the clock style based on the region you picked during setup or in the language settings. That is why a fresh device or a region change can switch your lock screen and status bar from 12-hour AM/PM display to 24-hour military time without any action from you.
The setting responsible for this sits inside the Date & time menu, separate from the region and language options themselves. Toggling it lets you keep any region selected while still choosing the clock style you prefer.
To change it, open Settings ➔ System ➔ Date & time ➔ toggle Use 24-hour format. Turning the switch on shows the 24-hour style everywhere on the phone, including the lock screen, status bar, and most apps. Turning it off returns the clock to the 12-hour AM/PM format.
This setting is global and affects the whole system, so you cannot set a different format for individual apps. If only one app still shows the wrong format, that app is likely using its own internal clock setting rather than following the system, and adjusting Date & time will not fix it.
Automatic date and time detection stays independent from this toggle. You can leave Automatic date & time enabled for accurate time zone updates while still manually controlling whether the hour count runs from 1 to 12 or 0 to 23.