For Merlot, and LadyLeft (& more), the old O Levels were grade 1 down to 6 (passes), and 7, 8, 9 as failures. I think I got a 1 and 2 in Maths and English, then a 3 and a couple of 4s for the five passes that you needed then to get a passable job. Or move on to 6th form / A levels. I was put in for eight exam subjects, but worked out that the work to try and pass that lot was too hard, so I gave up on two (Spanish & Eng Lit), intending to really try for the other six (so, should be OK to pass at least five?).
In fact I messed up the History paper, I'd been good at that subject right through school, but they managed to ask questions that were a battle too far (or, our teacher steered us towards the wrong topics?).
Back then, the CSE exam had recently arrived, for secondary school kids with lower hopes, I believe a grade one pass in a CSE was called equal to a grade 6 O Level?
I've met teachers over the last few decades who are quite convinced that modern GCSEs (as everything evolved) are nothing like as difficult as O Levels. And that A Levels have been watered down too.