Is there a difference? Or just to confuse us
That’s a very good question! Both forums are monitored by Arm and are public to the community. https://os.mbed.com/forum/mbed/ is the older site and is well known by customers and partners (and even linked to by our corporate home page). It is also dedicated to Mbed OS. The newer forums site (https://forums.mbed.com/) is for more than just Mbed OS as you will find Pelion and TLS areas so it is targeting a broader audience.
For OS-centric discussions right now we recommend using the older forums and for non-OS discussions please use the new forums.
-Ralph, Team Mbed