I was the guinea pig year for GCSE and got 4 As 3 Bs and a c.
I think some O levels would have suited me better, eg I am better at exams than coursework, so the B I got for my English Lit GCSE (100% coursework) might have equated to an A for O level.
But, I got Bs for Chemistry and Maths and feel sure I would have only got Cs if I had done them for O level.
I wonder whether the new, supposedly harder, GCSEs are actualy more like the GCSEs I took. My German GCSE in 1988 was much harder than my Italian GCSE in 2001, when I was allowed to take an English-Italian dictionary into the exams and got an A*. So my feeling is that GCSEs had got easier over the years and a reboot was necessary.
I did 8 GCSEs in my grammar school, my son's comprehensive school has also decided that most students will do 8, which indicates some sort of perceived parity with the old GCSEs.
I can't comment on O level because I didn't do any, although we did do a certain amount of O level questions as practice, as there weren't that many specimen GCSE papers available.