Secondary school exam in England, Wales and Northern Ireland from 1951 to 1986 (1,5)

I believe the answer is:

o level

(Other definitions for o level that I've seen before include "Qualification for 14-16-year-olds in the UK" , "British school examination" , "Basic level of a subject in English school" , "School exam" .)

I've seen this clue in The Guardian.

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