"Operating Systems" for microcontrollers such as this are frequently just schedulers as the Application typically contains drivers and hits the hardware itself. e.g. FreeRTOS.
Even those implement communication primitives e.g. locks, semaphores, mailboxes, etc. as part of the OS. Some of the larger ones also define a driver model of sorts (which often just codifies the structure of the first driver for that type of peripheral e.g. USB, Ethernet, CAN, I2C, SPI, GPIO).