O grade or Ordinary grade noun

In Scotland, (a pass in) a former examination generally taken at the end of the 4th year of secondary education (also adjective, often with hyphen)

O level or Ordinary level noun

Esp in England and Wales, (a pass in) a former examination generally taken at the end of the 5th year of secondary education (also adjective, often with hyphen)

O

1. (as a medieval Roman numeral) 11
2. /ō/ oxygen (chem)

Ō

(medieval Roman numeral) 11000

O¹ or o /ō/

1. The fifteenth letter in the modern English alphabet, fourteenth in the Roman, derived from Greek omicron (Ο, ο), with various sounds, as in note, not, for, work, son, do
2. One of the four blood types in the ABO blood group system
3. In a series of numbers, nought or nothing
4. (usu in pl) a spangle (obsolete)
5. Anything shaped like the letter O