Android O is still scheduled to be released for the OP3/3T, but according to a Q&A that OnePlus shared on its forums, you shouldn't expect any major software updates following it. These two were succeeded by the OnePlus 5, and after its release, there were talks about OnePlus giving software update to the OnePlus 3 and OnePlus 3T.
So, will they? OnePlus has said "yes" before, but this time, it has confirmed that it will be that last Android version upgrade for the pair.
Also on the outs after Android O is the open beta program for the OnePlus 3 and OnePlus 3T.
While the advent of Android O update is indeed a good news for the anxious OnePlus 3/3T owners, the confirmed dearth of updates, following that will surely discourage many. He further revealed that the Android O update will be rolled out along with security patches and individual app updates as well for the near future. It's worth noting that OnePlus has been pushing continuous updates through the beta program to test the new features before pushing a public release. OnePlus 3/3T are one of the best Smartphones of the year 2016 and there's no excuse to drop support for a device this soon. Buyers of the OnePlus 3T probably would have liked at least two years of software support.
As it pertains to the latest security and OxygenOS updates, the August patch will come out before the OxygenOS 4.5 update.