New Wear OS devices run two operating systems

Wear OS smartwatches have a dual-chipset architecture inclusive of a powerful application processor (AP) and ultra low-power co-processor microcontroller unit (MCU). The architecture has a powerful AP capable of handling complex operations en-masse, and is seamlessly coupled with a low power MCU.

The Wear OS hybrid interface enables intelligent switching between the MCU or the AP, allowing the AP to be suspended when not needed to preserve battery life. It helps, for instance, achieve more power-efficient experiences, like sensor data processing on the MCU while the AP is asleep. At the same time, the hybrid interface provides a seamless transition between these states, keeping a rich and premium user experience without jarring transitions between power modes.

Kseniia Shumelchyk on the Android Developers Blog

The new OnePlus Watch 2 is the first to use this new architecture, and the most interesting part is that it runs not one, but two operating systems: Wear OS, which is Android, running on the AP", and a smaller RTOS that runs on the MCU". In the case of the OnePlus Watch 2, the AP" is a Snapdragon W5, while the MCU" is a BES 2700, an ultra low power microcontroller.

I can't seem to find any information on this RTOS", but I'd really love to know what it's based on.