Oneplus Watch has a dual OS setup. Wear OS and a RTOS [Real Time OS]. The pre built watch faces are optimized for both scenarios , I assume and if say, AOD is enabled, Wear OS won't be running in the background. It would be RTOS and the watch face would still work. Third party watch faces might compel/force the watch to use Wear OS at all times, creating unnecessary load.
This dual OS is the reason why OnePlus can crank 3-4 days with AOD enabled when even GW Ultra will have trouble hitting that mark. Interestingly, Mobvoi has a different trick up it's sleeve. Their watches have a dual screen setup; with the low power display being used for stuff like AODs. This again make TicWatches last significantly longer than Pixel or Galaxy Watch.