Here's where OnePlus may start to fail in this comparison. While they have the better design, screen, specs and more, the camera is typically a sore spot.
Over the years the company has drastically improved the camera situation, but they still won't be able to compete with Google. In fact, we believe the single 12.3MP UltraPixel camera on the back of the Pixel 3 XL will be better than the dual camera setup of the OnePlus 6T, iPhone 11XS Plus, Note 9 and more. Google does some crazy stuff with the camera software that makes it the best camera around.
The new Night Sight Mode, Top Shot, and machine learning allows that single camera to take stunning photos no matter the situation. OnePlus can't top that.
With the OnePlus 6T the company is using the same camera as the OnePlus 6. And while that's fine, it can't compete with Google's camera. It still makes great photos that are better than anything from a few years ago, it's just not the best you can buy in 2018. For many, that's not a concern, but it's worth mentioning.
OnePlus does have a new Night mode that should help in low-light, which was where it struggled the most. For everything else the cameras are good enough for most situations.
Without talking numbers and specs, the Pixel 3 XL has the better camera. No doubt. But is that worth losing everything else and paying more? That's for you to decide.