Launched in 2016 with Android 6.0 Marshmallow and updated this 2017 to Android 7.1 Nougat, the OnePlus 3 and OnePlus 3T will receive in 2018 its latest version: Android O.
The Chinese company will not offer more than important updates during the first 18 months of device life.
This has been confirmed by its Product Manager Oliver Z. in a Questions and Answers from their support forums.
Android O will be the latest version of Android that will release for OnePlus 3 and OnePlus 3T, but will continue for a while more by posting security updates and updating their individual applications, but nothing from Android P.
With the OnePlus 3 released in June they will have offered two years of support, but with the OnePlus 3T released in November, their support will stay in 18 months.
It is not yet known when Google will launch Android P, but possibly before November 2018, with what we see that OnePlus already makes clear that they will not bother to update this device even though it has less than two years in the market.
When OnePlus 3 and 3T upgrade to Android Or the OnePlus Beta program will move to OnePlus 5, so your newest device will be the one that will begin to receive exclusive OxyGen OS updates.
Oliver Z. also confirms that his developers will now be focused in the first few months to get updates for the OnePlus 5, to solve the problems and to add new features in his new device, but they will still continue to launch a monthly update for the OP3 and 3T.