NuttX is one of the better MCU RTOS's I've encountered. It is designed and written by a team of experts, it comes from the private-sector (so they have to keep focused on things that matter), and the support is good. And it's open source.
www.nuttx.org
I'd like to port to MSP432. I would also like to integrate a low-power BIOS into it, because at present the scheduling architecture isn't well-suited to using the ultra-low-power sleep modes on ARM. This second task, I've already done in large part for OpenTag on STM32L, but integration is never plug-and-play with this stuff.
Ping me if interested! I'll set-up a GitHub repo momentarily. You should have an MSP432 Launchpad, as that will be the target board. If you've worked on NuttX, even better.