Dates are shown as ‘dd-mm-yyyy’ in my OmniFocus.
So I guess this is not a setting in OmniFocus, but rather the system defaults of your mac.
You might want to check System Preferences > Language & Region, click ‘Advanced’ and check the settings on the ‘Dates’ tab. My guess is that OmniFocus is using one of these date-formats. I’m not sure if it is possible to tell OmniFocus which one of these formats (short, medium, long or full) to use…
Remember when you change things here: other apps on your mac use these settings as well!