Android updates have never been very fast or consistent, but Google took some steps to improve the process several months ago. That's when it announced Project Treble, a feature of Android Oreo and later that makes OTAs modular. Devices that launch with Oreo will support Treble, but those updated to Oreo won't—at least mostly. OnePlus has confirmed on its forums that Treble won't be part of any current devices, including the new OnePlus 5T.
With Treble, devices will have a vendor implementation that manages all the hardware at a low level. Then, the Android framework plugs into that. Thus, OTA updates don't need to include new drivers for the silicon. That means devices can be updated faster and longer, but only the ones that launch with Oreo. Google updated the 2016 Pixels with Treble, but that was a special case. Essential, too, is reportedly working to make Treble a reality.
OnePlus launched its new flagship phone with Nougat, but some OP fans have been counting on Treble showing up when Oreo launches in early 2018. I've actually had people asking me about this on Twitter, and it was one of the first things OnePlus forum users inquired about when given a chance.
The answer is (unsurprisingly) no. None of OP's current phones will get Treble in the Oreo update. It's a bummer that a brand new phone like the 5T is left out of Treble, but the OnePlus 6 should have it. Given OP's aggressive release schedule, you should be able to buy that phone in a few weeks. I kid—it'll be like five months. OnePlus is rolling out Oreo to the 3 and 3T, and 8.1 is on the schedule too.
OnePlus held an AMA on its forums and explained the Treble situation further. Apparently, adding the necessary partition to phones in an update comes with a high risk of bricking devices. Thus, OP had decided not to risk it.
- OnePlus forums