Some recent leaks have pointed to a first-gen smartwatch from OnePlus, for which the OEM might develop a proprietary UI. However, a new interview with this company's CEO indicates that it will use Google's WearOS - then again, it may be heavily modified so as to fit in with its device ecosystem.
https://www.notebookcheck.net/The-OnePlus-Watch-is-likely-to-run-WearOS-after-all.510439.0.html