Here are the latest OxygenOS Open beta builds for the OnePlus 6 (Open Beta 21) and OnePlus 6T (Open Beta 13).

Changelog

• System
  • General bug fixes and stability improvements
  • Screen Recorder
  • Added FPS (Frames per second) option giving you more control over recording effects
• Weather
  • Improved scrolling experience for switching between pages
  • Weather cards for multiple cities can now be rearranged or deleted by tapping and holding on them
  • Optimized user experience by allowing you to use this app while app permission to access GPS location is disabled
• Phone
  • Improved animation effects when switching between pages
  • Improved experience with searching for contacts
  • Optimized Quick Responses for declined calls and supported editing Quick Responses

Please keep in mind that this is beta software. These builds are sometimes not as stable as official OTAs generally are. By installing this update, you accept the potential risks. 

Please note:

• If you have already flashed an Open Beta (you are currently running the latest Open Beta), you will receive this new build as an OTA.
• If you are not running open beta software and would like to, please refer to the flashing instructions and the full ROM on the downloads page here.
• Once you migrate to the beta path, you will continue to receive Open Beta OTAs. You will no longer receive the regular Official Stable OTAs.
• Moving back to the Official OTA path from the Beta path will require a full install and clean flash (full wipe of all data and cache).

Source OnePlus Community