OOB = Out Of Battery
Cartridge fires but bolt is not locked in battery. I believe this is the same as slamfire.
Not specific to open bolt (fullauto) guns, where trigger releases the bolt to feed and fire each cartridge.
"In firearms and artillery where there is an automatic loading mechanism, a condition can occur in which a live round is at least partially in the firing chamber and capable of being fired, but is not properly secured by the usual mechanism of that particular weapon. The gas pressure produced at the moment of firing can rupture the not fully supported cartridge case and can result in flame and high pressure gas being vented at the breech of the weapon, potentially creating flying shrapnel possibly injuring the operator."
http://en.wikipedia.org/wiki/Out-of-battery