Defunct academic qualification. Also known as a General Certificate of Education (GCE).
It all went horribly pear-shaped in the 1980s when someone decided that the (then) new Certificate of Secondary Education (CSE) was worth a C Grade O-level/GCE (or something along those lines), providing the hapless Guinea Pig student accrued enough smartie points. Wishful thinking really, as any employer worth his salt could spot a mong with a string of worthless CSEs.
Anyone with enough O-levels went to college to do A-levels. Those who could not be arrsed went in to the Armed Forces or tried to acquire one of the dwindling number of available apprenticeships.
Scots had O-Grades which were dispensed in toilet cubicles for you to wipe your arrse on. This, naturally, had the effect of making the content far superior to anything ever scribbled on a GCE paper - with the minimum of effort.