It could be considered broadly equivalent to the old English O-Level qualification and is the predecessor to the Standard Grade.
Along with its more advanced sibling, the 'Higher Grade', the O-Grade was the bedrock of the Scottish educational system for many years.
A good pass at O-Grade would normally enable a pupil to take the same subject at Higher Grade in his or her fifth (or possibly sixth) year, if so desired.
This did not always follow however, particularly as many more pupils left school at the end of their fourth year than is the case nowadays.
The Standard Grade has eventually been replaced by the National 4/5 qualifications, as part of the introduction of the Curriculum for Excellence.