Custom Babylon or middle-east stained glass pattern with the english letter O.  But this design could be used just for a graphic, rather than a monogram or logo with the letter O.  Make this reasonably large, as there are many small pieces, and the detail will be lost in the copper foil or lead came if it is too small.   Enlarging a pdf file (which is a vector format) by the quick print places is usually free, if you print it there with your large size output printer.