| Word root/ prefix | Root Meaning | Words based on the Root |
| Numer | Number | Enumerate - to mention a number of items on a list Numerology - the study of magical uses of numbers Numerous - a large number. |
| Oct/a/o | eight | Octagon - a figure with 8 sides and 8 angles Octogenarian - person in his or her 80s Octopus - sea animal with 8 arms. |
| Omni | all | Omnipotent - with all the power Omniscient - knowing all things Omnivorous - eating all foods. |
| Opt | best | Optimal - the best, the most desirable Optimize - to make the best of Optimum - the best something could be |
| Ortho | straight | Orthodontist - a dentist that straightens teeth Orthopedic - a doctor concerned with the proper alignment of the bones Orthography - the correct way of writing. |