This is just an educated guess but possibly O'Connor Jewellers who used to operate a workshop and showroom at 133 Harold's Cross, Dublin 6W. This family company was in business in 1960 according to one post I read and probably well before then too. It ceased trading in late 2010, apparently due to the recession.
http://dublin.ratemyarea.com/places/o-c ... lers-13916However, I don't know if the company used O.C as its maker's mark.