Android 9 Pie is currently available for the OnePlus 5, 5T, 6, and 6T, while the OnePlus 3 and 3T users have been waiting for this update for quite some time now. OnePlus did promise that the update is coming soon, and the company has just made it available, but only in beta form, for now.
The company shared the news via its official forums, and as it's usually the case, this indicates that the stable build is right around the corner. Do keep in mind that the update will not arrive to your phone OTA (over the air), even If you do have OnePlus' official beta software installed on your phone.
OnePlus notes that you will have to download and install the update yourself if you using beta build already, links are provided below the article. Before you proceed, though, note that your entire data will be wiped this time around, if you're on the latest Open Beta version 180712. OnePlus says that this will happen as there will be a version transition from Open beta to the stable version soon.
If you're running a stable build of OxygenOS, on the other hand, you will need to flash this update manually, but please do note that it is always possible you will brick your phone, so if you don't know exactly what you're doing, it's better you wait for the stable build. If you're willing to go through with it, instructions can be found in the link provided in the first paragraph.
As was the case with OnePlus' previous smartphones which received Android 9 Pie, this update brings quite a few changes compared to Android 8.1 Oreo which is currently running on the OnePlus 3 and 3T.
This update makes quite a few changes in OxygenOS, though it's not a complete overhaul. The most notable change that you'll notice is probably the overview / multitasking menu, as it now features a horizontal list of cards, not a vertical one. This setup is less productive, but it is what Google presented with Android 9 Pie, so OnePlus opted to adopt it. Some changes in the notification shade are also visible.
This update will also bring Android security patch for April 2019 to both devices, and a new Do Not Disturb mode with adjustable settings will also be included in the settings. Pretty much every feature that Google announced as part of Android 9 Pie is available here, including Adaptive Battery and Adaptive Brightness features, which should help out with battery life as well.
Text notification mode has been added to OnePlus' Gaming mode 3.0, and also a notification for 3rd party calls. Deep integration with Google Duo is being pushed to the device as well via Android 9 Pie update, and the same goes for Google Lens mode integration for OnePlus' camera.
All in all, Android 9 Pie does resemble Android 8.1 Oreo in terms of design, quite a bit, but some changes have been made in that regard, even though most changes have been made under the hood. In any case, Android 9 Pie should bring new life to the OnePlus 3 and 3T, even though both devices are still performing quite well.