This General Public License does not permit incorporating your program into proprietary programs.
Here, we understand that 'your program' refers to OnionOS.
PS: By priority, I mean to say that I much prefer that all efforts be toward the Omega2 delivery, than pointless license evidence.
But thanks for the link you provided. It makes a nice distinction between Free Software and Open Source Software. I never devoted any attention to the difference. So obviously, OnionOS is Free Software (where the source code has yet to be fully disclosed to be able to build a nice Omega and/or Omega2 firmware painlessly ;) ).
PS2: My preference would be Open Source Software, but that is clearly impossible because of the OpenWRT's license.