OnePlus has merged its research and development department with Oppo. OnePlus will continue to operate as a separate brand under Oppo, but its smartphones will feature a new UI that will bring the best of Oxygen OS and Color OS. This OS will also be available on Oppo phones going forward.
Additionally, OnePlus CEO Pete Lau has also revealed how the naming scheme of the OnePlus smartphone family works.

OnePlus R Confirmed to be Entry Level Flagship Series

All phones priced above $500 will be a part of the ultra-premium flagship lineup with cutting-edge technology. These phones will be widely available in the global market. The other lineups will not be available everywhere around the globe and will target specific markets with different price ranges instead.
The OnePlus R, Nord, and Nord CE will fall between the $300 – $500 price range. OnePlus R phones will be the entry-level flagships, the OnePlus Nord family will include premium mid-range phones, and the Nord CE will offer entry-level and mid-range phones.
As a recap, the OnePlus 9R is cheaper than the OnePlus 9 and is only available in India and China. The OnePlus 9 RT is also expected to release in these markets with a cheap price tag. The OnePlus Nord CE is available in India as well as Europe.
The Nord N series will feature the cheapest OnePlus phones priced between $100 and $300. So far these phones have only been released in the US.