Few days back, OnePlus released Android 8.0.0 Oreo based OxygenOS 5.0.2 stable build for OnePlus 5T, but it was suspended immediately due to stability issues. OnePlus 5T again updated with the stable OxygenOS 5.0.3 build. This new build does not have any major changes from the previous 5.0.2 build, except fix for system stability issues.
If you have already updated to OxygenOS 5.0.2, you will get the new 5.0.3 OTA (over the air) update, which weighs around 39MB. You can also download and flash the full OTA update file (download link below) manually, if you cannot wait for the OTA. However, we recommend OTA update, as it is safe and simple.
The new OxygenOS 5.0.3 update includes all the features of previous OxygenOS 5.0 2 build, such as updated Launcher, Gallery app, Weather app, and File Manager, CPU security patch, and new design for Quick Settings along with new system stability fixes.
Check for the OTA update manually by going to Settings > System Updates > Check for OTA updates. If update found, download and install right away.
If your prefer manual installation, then download the OxygenOS 5.0.3 full OTA zip file (1.6GB) from here and install it on your OnePlus 5T.
After downloading the firmware, you may follow this installation guide to know how to flash the firmware manually on your phone.
After installing and using OxygenOS 5.0.3 on OnePlus 5T, please let us know if you notice any improvements to the stability of the system in the comments. (Source)