Out Of Bounds
A place you are forbidden to visit or frequent (it is usually where the good bars are). Anywhere marked as out of bounds is usually where you will find most of the regiment (especially RHQ) at the first opportunity.
The bars and clubs that are 'OOB' are usually portrayed on orders as "sordid fleshpots which no decent thinking person would want to visit" - hence why they are irresistible to squaddies.
'Oob!', is also the noise you make when hit sharply in the testicles by a cricket ball. This is generally followed by twenty minutes or so of quiet contemplation, accompanied by a little cold sweating, curled up into a foetal position.