Hi,
This is not a Swisslog problem. ASCII codes 216 or 248 are represented like this (there is no oval slahed O like you want in the ASCII table). Swisslog offers the possibility to replace all zeros by a selected character of the international ASCII table. Characters 216 or 248 are the most approach character of slashed 0.
However you can install any of the fonts available in the following site and use it in your QSL card format:
http://www.g0hwc.com/ham_radio_fonts.html
All font in this site represents the slashed zero as you want and you don't have to use the "Print 0 with a bar" function.
Best 73