OxygenOS (OOS) is OnePlus' well-received Android-based ROM for the global units of its phones. It has a Chinese-market counterpart called HydrogenOS (H2OS). However, the OEM has reportedly informed its users that it is to be discontinued in favor of OPPO's ColorOS instead.
https://www.notebookcheck.net/OnePlus-is-reportedly-switching-from-HydrogenOS-to-ColorOS-in-China.528159.0.html