Years ago, when I took O level music, that was accepted by the ABRSM instead of grade 5 theory if you wanted to take a higher instrumental grade. That might be what the person was thinking of. It isn't the case now - you've got to have Grade 5 theory (or jazz or practical musicianship - is that right?).
O level music involved a lot of music theory that isn't covered by GCSE music today. GCSE is, however, much more wide-ranging and covers many more musical styles and genres - it just doesn't have the theory content of the old O level. My daughter's taking it - she recorded her group performance today - I think it went OK!