In a post in the OnePlus forums, the company highlighted what OxygenOS 5.0 is bringing to the table. It bumps the Android version to 8.0 Oreo, bringing latest features such as Picture in Picture (PiP), Parallel Apps, Auto-fill, Smart text selection and a new Quick settings design. The security patch has only been upgraded to September. Just after five months after OnePlus 5's launch, OnePlus incorporated pretty much the same innards to its 5T smartphone.
From a common user point of view, why one will buy OnePlus 5 in place of OnePlus 5T when both of them same costing the same and the fresh one is coming with upgraded software and others. "We will continue to follow up on the feedback from the community and roll out incremental updates". You shouldn't install the update in case your OnePlus 3 or 3T has less than 50% charge remaining. Starting with the launch of the OnePlus 5T, OnePlus will no longer target benchmarks with performance modes that do not represent day-to-day usage.
You may have seen blogposts from OnePlus confirming the arrival and Rollout of the OxygenOS 5.0, which is going to be available for the company's OnePlus 3 and 3T smartphones.
Long story short, the OnePlus 3 and OnePlus 3T are already getting Android 8.0 Oreo, while the newer OnePlus 5 and OnePlus 5T are stuck with Android Nougat until 2018. As the competitors provide better specifications, they had to update their smartphone to sustain in the market. OnePlus devices used to be the go-to choice for mid-range devices with an affordable price tag which it still is but has to innovate in terms of design and value the phone delivers to its buyers.