Out of Band(OOB) can mean something like trying to deduce the underlying code of an application by listening to the influence on an AM radio the electronic noise emitted by the CPU as it runs the program.
This can also be called a side channel attack
I have a friend and like me he publishes a web page. One time he typed into the location field of a browser inresco.org/Hi dfy, its me Peter
The part after the slash after the domain name does not describe a file in my document root so it produces an error
I can type thesprawl.org/hi peter, its dfy
and likewise it will show up in his server logs as an error

OOB came about as I watched the behavior of web crawlers. After crawling through a directory not the root directory they/it would attempt to access 'default.html' and would produce an error until I created a file with that name which often be one of the nodes of Crowther and Woods vending machine labyrinth from the computer game 'The Colossal Cave' which ran on UNIX machines in the early 80's
When learning html starting about 1995 I recreated in html the vending machine labyrinth from my notes from playing this game. Finding your way into this labyrinth can happen if you metaphorically fall through a trap door while browsing my web site which has 10,000 documents in my document root, about 2000 of them .html files.

Out of Band from Korea 1
Out of Band from Korea 2