There are a lot of realtime operatings systems out there right now, competing for a spot on the IoT hypetrain:

- the mentioned RIOT OS (https://riot-os.org/)

- FreeRTOS (https://www.freertos.org/) and Amazon's version of it (https://aws.amazon.com/freertos/)

- Zephyr OS (https://www.zephyrproject.org/)

- MyNewt (https://mynewt.apache.org/)

- Mbed OS (https://os.mbed.com/)

and more... Makes it really hard to choose one!