Though the rumors claimed that OnePlus 3T will run on Android Nougat out of the box but in reality, initially, OnePlus 3T will run on Android 6.0.1 Marshmallow and not Android 7.0 Nougat.
OnePlus 3T will get Android 7.0 Nougat update in December this year. OnePlus 3 update will also come in December 2016.
OnePlus says that both the handsets will have update schedules. It seems that OnePlus 3 and OnePlus 3T will receive simultaneous updates unless one of the devices is found faulty earlier.
However, it is still unclear whether or not there will be any Android Nougat betas for OnePlus 3T in this month as there will be one for OnePlus 3.