IGCSE Mark Scheme O/F?

Question

Hi in University of Cambridge International Exams (like IGCSE) in the mark scheme it has a O/F. Particulary in Accounting. What does O/F Mean?

Here is examples: http://www.cie.org.uk/docs/dynamic/1222.pdf

Answer 1

i have no idea, maybe something to do with frequency?

if you know what c/f is maybe that'll help.