OnePlus has a somewhat spotty record when it comes to updates. The original OnePlus One was updated from Android 4.4 to 6.0, but the OP2 and OPX are still stuck on Marshmallow. Pete Lau, founder and CEO of OnePlus, has confirmed that the OnePlus 3 and 3T will get Android O.
A lot of you have been asking, so I'm proud to say Android O will come to OnePlus 3 and 3T.
— Pete Lau (@petelau2007) May 31, 2017
The OP3/3T took about four months to get Android 7.0 after that update was released, so I would expect a similar time frame for Android O. Hopefully OnePlus will share a more detailed plan once Android O is released.