Index of letter O
| -o | Marking informally shortened or slang nouns. |
| -o- | A linking vowel. |
| o-1 | See ob- |
| o-2 | See ortho- |
| oario- | See ovario- |
| ob- | Towards, to, on, over, or against. |
| occipito- | The occipital bone. |
| -ock | Forming nouns. |
| -ocracy | See -cracy |
| octo- | Eight; having eight. |
| oculo- | The eye or the sense of vision. |
| -ode1 | Like; of the nature of. |
| -ode2 | A conductor of electricity. |
| -odont | Teeth; toothed. |
| -odontics | Specialisms in dentistry. |
| odont(o)- | A tooth or teeth. |
| -odynia | Pain. |
| oen(o)- | Wine. |
| oesophag(o)- | The oesophagus. |
| of- | See ob- |
| -ogen | See -gen |
| -ography | See -graphy |
| -oholic | See -aholic |
| -oic | A carboxyl group or its derivative. |
| -oid | Like; resembling. |
| -oides | See -ides |
| -ol | An alcohol or phenol. |
| -ola | Diminutives; trade names; humorous or dismissive formations. |
| -olatry | See -latry |
| -ole1 | Organic chemical compounds. |
| -ole2 | Forming nouns. |
| ole(o)- | Relating to or containing oil. |
| -olidine | See -idine |
| olig(o)- | A small number; having few. |
| -ological | See -logical |
| -ologist | See -logist |
| -ology | See -logy |
| -oma | A tumour or other abnormal growth. |
| ombro- | Rain; moisture. |
| -ome | Having a specified nature. |
| -ometer | See -meter |
| -ometry | See -metric |
| omni- | Of all things; in all places. |
| omphal(o)- | The navel; the umbilical cord. |
| -on1 | Subatomic particles or quanta; molecular units. |
| -on2 | Inert gases. |
| onco- | A tumour. |
| -one | Various chemical compounds, especially ketones. |
| oneir(o)- | Dreams or dreaming. |
| -onic | Organic acids. |
| -ont | An individual or cell of a specified type. |
| onto- | Existence or being. |
| -onychia | A condition of the nails. |
| onycho- | The nails. |
| -onym | A name. |
| oo- | An egg or ovum. |
| -oon | Forming nouns and derived verbs. |
| oophor(o)- | The ovary. |
| op- | See ob- |
| -opedia | See -pedia |
| ophi(o)- | A snake. |
| ophthalm(o)- | The eyes. |
| -opia | A visual disorder. |
| opistho- | Behind; to the rear. |
| -opsia | A defect of vision. |
| -opsis | Observation or perception; likeness. |
| -opsy | Medical examination. |
| opto- | Vision; light. |
| -or1 | Someone or something that performs a given action. |
| -or2 | Forming nouns denoting an abstract state or condition. |
| -or3 | See -ior |
| -orama | A display or spectacle. |
| orchid(o)- | Testicle; orchid. |
| organo- | Bodily organs; organic chemical compounds. |
| -orial | See -al1 |
| -orium | A place for a particular function. |
| ornith(o)- | A bird or birds. |
| oro-1 | Mountains. |
| oro-2 | The mouth. |
| ortho- | Straight; correct; upright. |
| -ory1 | A place having a particular function or character. |
| -ory2 | Relating to or involving an action. |
| os- | See ob- |
| -oscope | See -scope |
| -ose1 | Having a specified quality. |
| -ose2 | Sugars and other carbohydrates. |
| -osis | Abstract nouns of action, state, or process. |
| -osity | Forming nouns. |
| -osmia | An abnormal condition of the sense of smell. |
| osmo- | Osmosis. |
| oste(o)- | The bones. |
| -ostomy | See -stomy |
| -otic | Forming adjectives and nouns. |
| oto- | The ears. |
| -otomy | See -tomy |
| -our | Forming nouns. |
| -ous | Forming adjectives. |
| out- | Surpassing; external; away from. |
| ovari(o)- | An ovary. |
| over- | Excessively; extra; outer; above. |
| ovi- | Eggs, ova. |
| oxa- | See oxy-2 |
| oxy-1 | Sharp, acid. |
| ox(y)-2 | Oxygen. |
| -oyl | See -yl |
| -ozoa | See -zoon |
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