When answering the question: “Which was harder, O-Levels or GCSEs?” there are two different answers depending on what precisely you are asking about.
If you are asking: “Was it harder to get an A on O-Levels compared to GCSEs?” the answer is: Yes, it was harder to get a high grade on O-Levels rather than GCSEs. This is because grades for O-Levels were norm-referenced meaning only a set number of students could ever achieve the highest grade. Given that only students who had showed a high level of proficiency at the age of 11 were entered for the exam, the competition to be in that top percentage was particularly fierce and so an A was a (relatively) rare accomplishment.
If you are asking: “Was it qualitatively more difficult to complete an O-Level exam compared to a GCSE exam?” or “Did you need to know more to get an O-Level than a GCSE exam?” then the answer is more complicated. Having looked at O-Level papers the content is no more challenging than what is expected of GCSE candidates – in fact in some places it seems rather easier. The O-Levels have far more choice in each paper, more marks given for conjecture, and the format of questions requires much less interpretation than today. Of course wise question choice and conjecturing are skills in themselves so I shall not argue that GCSEs are *harder* but I do think there is enough evidence to shows O-Levels were not qualitatively more difficult either. Hence, if you are asking are GCSEs easier to DO than O-Levels my answer would be: No.