Perhaps your gauge has mutated??? O.L might just be a faulty letter I in the middle reminding you 5000km have passed and you need to change the Rohloff nectar in that hub of yours 
It measures the approach of The Day of the Triffids. They're turning the corner down the block.
It is odd, I have a gauge like Matt's, though mine is branded by someone else, and it is probably thirteen plus years old, and it has required a new battery so rarely that I can't remember when I last changed the battery. Utterly reliable.
Schwalbe used to pack a very simple complimentary gauge with their tubes, just an outer tube with a sliding rod inside, the exposed gradations of which indicate the pressure, near enough for roadside work. It weighs next to nothing. I used to carry one in each bike's toolkit, but since I electrified and weight isn't a consideration any more, I carry the electronic gauge.
Some thoughts: the OL may be a default error condition that has nothing to do with the effectiveness of the gauge. Try seeing if it measures anything, and if it measures what you're expecting to see, then it still works. If not, cycle it through its measuring scales by holding down the button a few seconds, and if it cycles, try measuring pressure on each of the scales. Also, if you're cleaning contacts, and you can reach it, check that the contact under the button is clean, as well as the battery contacts which you've been advised already.
