OnePlus
Lock Screen Notification Display Options auf OnePlus
OnePlus phones running Oxygen OS separate two things: whether a notification arrives at all, and how much of it appears on the lock screen. The lock screen visibility is controlled by a dedicated system setting, independent from each app's own notification permission.
By default the lock screen can show full previews, meaning sender names, message text and even image thumbnails. That is convenient at home but risky on public transport or in shared offices, since anyone glancing at the screen sees the content.
To change this, open Settings ➔ Apps & notifications ➔ Notifications ➔ Notifications on lockscreen. From there choose Show important and other notifications for full previews, Show important notifications only to hide less critical alerts, or Don't show notifications to keep the lock screen clean.
Choosing Don't show notifications does not silence the phone. Sounds, vibrations and the notification bar still work as usual, only the lock screen preview disappears.
If you only want to hide previews for one sensitive app rather than everything, check that app's own notification settings first, since some apps offer their own content-hiding toggle that works alongside this system-wide setting.