Pete Lau has confirmed that the OnePlus Watch will not run Wear OS, despite offering hints to the contrary last year. Instead, OnePlus has based its first smartwatch on RTOS, apparently because it provides a smooth and reliable experience combined with long battery life.
https://www.notebookcheck.net/Pete-Lau-confirms-that-the-OnePlus-Watch-will-not-run-Wear-OS.528173.0.html