| orthogonal | orthogonal |
| orthogonalität | orthogonality |
| Orthogon | rectangle |
| orthognath | orthognathic orthognathous |
| Orthodoxie | Orthodoxy |
| Orthoklas | orthoclase |
| Orthofoto | orthophoto |
| orthodox | orthodox |
| Oktogon | octagon |
| trigonal | trigonal |
| tetragonal | tetragonal |
| optional | optional |
| Original | original |
| original | original |
| Orthonitrat | orthonitrate |
| Orthorexie | orthorexia |
| Ortgang | bargeboard gableboard verge vergeboard |
| Orthoepie | orthoepy |
| Orthognathie | orthognathy orthognathysm |
| Orthografie | orthography spelling |