What options do exist in that space as an OS? Only thing that I personally know is Contiki, which people seem to either really love or really hate. (This is emphatically not my area of expertise, incidentally, so if I'm even misunderstanding how Contiki fits in here, I apologize.)
NuttX? http://www.nuttx.org
On CM3/4 CPUs I typically have been shipping with FreeRTOS based solutions. I've done this with LPC2478/1788, and I'm now working on a similar Kinetis K60 architecture.
It's a nice O/S, but doesn't have a real solid application layer on top of it. For that I've sometimes used FDI's uEZ system, but it also lacks a lot of the components I need for real IoT node work (Radio drivers, TCP, SSL, and as a stretch I could really use MQTT or AMQP).
Sigh.
Please stop saying "connected devices/IoT" and then require a desktop-class SoC to make it happen. This is a hub, not a node.
I need stuff that runs on Cortex-M with sub-Megabyte memory maps. I need my bill of materials to be an order of magnitude cheaper than a Raspberry Pi or Beaglebone solution. And no, I'm not using Weave or Lil' Droid or whatever Google is calling it now.