The recent rumors reveals that OnePlus is planning to launch the OnePlus 6 as early as March of 2018. The launch is said to be sometime in the middle of March, with the phone becoming available by the end of the month.
As for the specs, the OnePlus 6 will be powered by the Qualcomm Snapdragon 845 chipset and it will feature an underglass fingerprint sensor. As many prefer the sensor on the front and an underglass fingerprint sensor would make things easier as it can easily be installed on the front without having the sacrifice the full-screen design.
It might be too early to say that the above information is going to be 100% true, so take this as a pinch of salt for now.