OnePlus
Set a Timer in the Clock App auf OnePlus
The Timer function inside the OnePlus Clock app runs a fixed countdown and plays an alert sound once the time hits zero, unlike the Stopwatch, which only counts upward and has no built-in alarm.
To set it, open Open the Clock app and tap on Tap on Timer. Scroll the wheels to pick Choose Hour / Minute / Second, then confirm with Set the timer.
Once the value is locked in, tap Tap on Start to begin the countdown. The timer keeps running in the background even if you switch apps or lock the screen, and the alert sound still fires when it reaches zero.
Need to interrupt it? Tap Tap on Pause to freeze the countdown without losing the remaining time. Resume whenever you are ready, or tap Tap on Reset/Delete to clear the value and start over with a fresh time.
If the timer sound never plays, check that media volume is not muted. The timer alert usually follows the media or alarm channel rather than the ringtone volume.