This letter represents an eye, and is called in Hebrew ain (an eye).
The fifteen O's are fifteen prayers beginning with the letter O (See Hora Beatissima Virginis Mariae.)
An Irish patronymic. (Gachc, ogha; Irish, oa, a descendant.)
in Scotch, means “of,” as “Tam-o'-Shanter.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894