According to David Y on OnePlus Official forums:
https://forums.oneplus.net/threads/will-op3-and-3t-get-android-o.498382/page-8#post-16580894First, my answer to the question on the thread title:
Yes. OnePlus 3/3T will get Android O. OOS team are working on it, even new community app (not public yet) just finished the compatibility work for Android O last week.
Here are some update I'd like to share with you guys:
Currently the OOS team are focusing on the OBT program and Android O for OP3/3T, also stable updates for OP5. Next stable update for OP3/3T was planned, will be a minor update updated Android security patch level to August.
I will keep you all in the loop once I got some latest updates.