| OA | Oberallgäu |
| OAL | Ostallgäu |
| OB | Oberhausen |
| OD | Stormarn |
| OE | Olpe |
| OF | Offenbach |
| OG | Ortenaukreis |
| OH | Ostholstein |
| OHA | Osterode am Harz |
| OHV | Oberhavel |
| OHZ | Osterholz-Scharmbeck |
| OK | Ohrekreis |
| OL | Oldenburg |
| OPR | Ostprignitz-Ruppin |
| OS | Osnabrück |
| OSL | Oberspreewald-Lausitz |
| OVP | Ostvorpommern |