O-level is pointless, the hyphen is misused.
O' level follows the rule of an apostrophe standing in for something omitted but is, nonetheless, rather clumsy.
The thread title is fine and O level is fine, and that was the nomenclature ascribed to it at its inception.
If one really wanted to be so pernickety about apostrophes for abbreviations then shouldn't it be O'level's, in which the second apostrophe is not, as it might appear, the common "greengrocer's apostrophe" misused for a plural but to indicate the missing word "examination"?.