[iberator]

Run Unix v6 on it :) 16 bit and works with like 80kb of ram

[sargstuff]

EPS32-OS : https://github.com/ESP32DE/Boot-Linux-ESP32S3-Playground

[dredmorbius]

Fuzix might be another option: <https://fuzix.org/>.

Works on the RISC-V 32-bit processor, sometimes.