The last O levels prior to introduction of GCSE were sat in 1988 so that cohort took their A levels in 1990.
I took GCSEs 1988, which was the first year. There were some trials in certain areas the year before, I think.
We didn't really know what the syllabus was except by what the teacher taught us as we went through the year. Now, I'd look it up online. You were stuck with whatever your teacher told you and the text book you were told to use. You could get revision guides, but most people didn't. I think now it's good practice to be more upfront about learning objectives and expectations.
In my first A-level French lesson, we did an old O-level paper, to start getting back into the swing of things, and there were some grammatical points we hadn't covered at GCSE. That was 35 years ago, though, and syllabuses will have changed more than once since (never had to learn vocab for mobile phones, Internet, WiFi and so on back then!) And I don't know if anything was introduced that hadn't been covered before.
For subjects like languages, you had to hope that someone kind would be going on holiday to France/Germany/wherever and could bring back a copy of a newspaper. Living on the south coast, I could sometimes get long wave to tune into French radio. Nowadays, I can order foreign language books online, look up YouTube videos, get DVDs, tune into Internet radio, join Internet chat groups, get any amount of stuff to read... as well as find all sorts of resources to explain grammatical points in different ways, if your teacher failed to get you understanding it.
There's also more about technique. We were told essays should have a beginning, middle and end, and that was about it. Exam technique was, read the full paper and the requirements, check the points to give an idea of the length of answer, and plan your time. (Oh, and for maths, always show your working.) That was pretty much it for everything. We only ever saw past papers in mock exams. I've done language exams as an adult, and I had access to almost every past paper for those. I think exam technique always was important, but we weren't really taught it.