The data saving mode prevents installed apps and services from sending and receiving data in the background in order to control the available data volume.
In addition, the automatic download of pictures or videos can be restricted, depending on the app, so that the content must first be tapped before it is displayed.

Note: When using the data saving mode on the OnePlus smartphone, it is recommended that app exceptions be enabled for Messenger services, as messages may be delayed in loading.

Android 10 | OxygenOS 10.0
Data saver -  Open the  Settings
Data saver -  Tap on  Wi-Fi & network
Data saver -  Tap on  SIM & network
Data saver -  Tap on  Data Saver
Data saver -  Enable or disable  Use   Data Saver
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on SIM & network
  4. Tap on Data Saver
  5. Enable or disable Use Data Saver

OnePlus Instructions