OxygenOS 9.0.5 over the air update starts rolling out now. OnePlus 5 and 5T users need to wait a few hours or sometimes even a few days to get the download notification on the phone. Alternatively, download and install the OTA zip file manually. Below is the direct link to download the firmware.
What is new? System
Updated Android security patch to April 2019
Phone
Improved stability for the Phone app
Parallel Apps
Fixed issue with parallel apps showing primary account Fixed issues with download pictures in parallel WhatsApp
Gaming mode
Fixed couldn’t receive third-party apps video call issues
A few days back OnePlus 6 and 6T got a stable update as OxygenOS 9.0.5 and OxygenOS 9.0.13 with March 2019 security patch and a few bug fixes. In addition, OnePlus recently released first public beta of Android 9 Pie for OnePlus 3 and 3T.
Download OxygenOS 9.0.5 OTA for OnePlus 5/5T
In case you want to install the update manually, then download the full firmware from below link. You can also use a VPN app to download the OTA by setting the location to Germany or Canada. Oxygen OS 9.0.5 OTA for OnePlus 5 [here] and OnePlus 5T [coming] After downloading the full OTA firmware zip file, users may follow this installation guide to update the phone to latest OxygenOS version manually. (source)