Most people don’t know this, but OnePlus is owned by BBK Electronics, a major Chinese technology man

Earlier this year when the OnePlus 5 was released, people noticed that it was completely identical to the Oppo R11’s design, although the latter was 0.5mm thinner. While the specs may have varied, it doesn’t seem like OnePlus did much in the way of product design, and it looks like that trend will continue.
Hopefully one of these days OnePlus and Oppo won’t have to share a design language and can build distinct devices, but that time isn’t here just yet.