'O' levels were GCSE's ... General Certificate Of Secondary Education. I had 7 in total, 6 in Mathematics and 4 in English.What's a GCSE? I only remember 'O' levels. Proper exams mind you, not like today.
Then you had CSE's ... Certificate of Secondary Education. In my days I believe a grade 1 CSE pass was equivalent to a grade 5 'O' level pass.
Anything below these and you had a choice of either going up a chimney, serving as a deckhand on a slave ship or Newgate prison... ahhh Happy Days