Currently, the OP3/3T receive updates once every 1-2 months. An open beta program, which you can sign up for, allows access to experimental features and new releases that OnePlus and it's community is working on. This program, says Oliver, will now focus on the OnePlus 5.
While Android O will be the last major OS update for the OP3/3T, Oliver does promise that the devices will continue to receive updates for the "foreseeable future".
As Wccftech points out, the OP3 is only 14 months old and the 3T a mere nine months old and it's surprising that support for the devices will end so soon. Android O is expected to launch before the end of 2017, that's just a few months away.
For comparison, Apple is still supporting ancient devices and the iPhone 5S, released four years ago in 2013, is expected to get the iOS 11 update this year.
In the course of the QnA, Oliver was also asked about some OP5 software features making their way to the OP3/3T via an update. Oliver said that this was already happening and that features that were tied to hardware updates couldn't be ported. As an example, he explained that the OP5's reading mode couldn't be ported because the OP3/3T didn't have an RGB light sensor needed for the feature to function properly.