os_config.json is used by NOOBS, and the message you are seeing comes from
/etc/init.d/apply_noobs_os_config. This is left default enabled but I would presume it disables itself after the first run; i.e., somewhat like
raspi-config it is run as a once only service. I'd also presume on a non-NOOBs system it does nothing.
What is it used for?
You'll find some examples if you search online but it mostly appears to be some basic stuff, e.g., this is from here (which is 3+ years old):
"description" : "An XBMC media center distribution for Raspberry Pi",
"flavour" : "RaspBMC",
"imagefolder" : "/settings/os/RaspBMC",
"keyboard" : "gb",
"language" : "en",
"partitions" : [
"release_date" : "2013-08-20",
"videomode" : 0
Some of this is likely for the NOOBS menu, and some of it maybe passed as options to the kernel or init -- in any case the general idea is pretty clear. I would not bother investigating further as I doubt this system will suit your purposes well.
I'm trying to customize an image and that sounds like something I should look at
This is ambiguous, but presuming you just want to automate installation, have a look around here for questions referencing puppet or chef (if you search separately, you will find more things). I notice this references a chef spin-off/fork/plugin/module (?) specifically for the Pi.
Personally, I find it easiest to just script that kind of thing myself, although this is no doubt dependent on how well you understand the OS, etc.