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| Ogle | stare at; observe in an obvious manner |
| Olfactory | concerned with the sense of smell |
| Ominous | threatening |
| Omnipotent | all-powerful |
| Omniscient | all-knowing |
| Onerous | burdensome; hard to undertake |
| Onus | burden |
| Opaque | does not let light through |
| Opulent | wealthy; rich; magnificent |
| Ordain | 1. destine; 2. confer holy orders on a priest |
| Ornate | highly decorated |
| Orthodox | conventional |
| Ossify | 1. turn to bone; 2. become fixed and rigid |
| Ostentatious | showy |
| Oust | push out of a position |
| Overt | obvious; not hidden |
| Overwrought | worked up; in an emotional state |
| Palatable | good to eat; agreeable |
| Palisade | fence made of posts |
| Palliative | a remedy that improves but doesn't cure |