[DanBC]

An O level is an obsolete English qualification. They got replaced by GCSEs.

The system used to be O levels ("ordinary") for high achievers, and CSE ("certificate of secondary education") for the rest. Both of those were replaced in about 1986/1987 with GCSEs. These are taken by school children at age 16 at the end of their secondary education.

They are followed by A (advanced) levels or other further education college course.