1. Oasis - a shelter serving as a place of safety or sanctuary
2. Obdurate - showing unfeeling resistance to tender feelings
3. Obedient - dutifully complying with the commands of those in authority
4. Obeisance - bending the head or body in reverence or submission
5. Obfuscate - make obscure or unclear
6. Objective - the goal intended to be attained
7. Obligation - the state of being bound to do or pay something
8. Oblique - not direct, explicit, or straightforward
9. Oblivious - lacking conscious awareness of
10. Obloquy - state of disgrace resulting from public abuse
11. Obscure - not clearly understood or expressed
12. Obsequious - attentive in an ingratiating or servile manner
13. Obsolete - no longer in use
14. Obstinate - marked by tenacious unwillingness to yield
15. Obstreperous - noisily and stubbornly defiant
16. Obstruct - block passage through
17. Obtrusive - undesirably noticeable
18. Obtuse - slow to learn or understand; lacking intellectual acuity
19. Obviate - prevent the occurrence of; prevent from happening
20. Occlude - block passage through
21. Occult - supernatural practices and techniques
22. Occupy - live in (a certain place)
23. Odious - extremely repulsive or unpleasant
24. Odium - hate coupled with disgust
25. Odoriferous - having a natural fragrance
26. Odyssey - a long wandering and eventful journey
27. Offend - cause to feel resentment or indignation
28. Officious - intrusive in a meddling or offensive manner
29. Offset - compensate for or counterbalance
30. Ogle - stare or look at, especially with amorous intentions
31. Olfactory - of or relating to the sense of smell
32. Oligarchy - a political system governed by a few people
33. Omen - a sign of a thing about to happen
34. Ominous -threatening or foreshadowing evil or tragic developments
35. Omit - leave undone or leave out
36. Omnipotent - having unlimited power
37. Omnipresent -existing everywhere at once
38. Omniscient - knowing, seeing, or understanding everything
39. Omnivorous - feeding on both plants and animals
40. Onerous - burdensome or difficult to endure
41. Onomatopoeia - using words that imitate the sound they denote
42. Onslaught - a rapid and continuous outpouring
43. Onus - a burdensome or difficult concern
44. Opalescent - having a play of lustrous rainbow colors
45. Opaque - not transmitting or reflecting light or radiant energy
46. Operative - a person secretly employed in espionage for a government
47. Opiate - a narcotic drug
48. Opponent - someone who offers resistance
49. Opportunist - a person who places expediency above principle
50. Opposition - a body of people united against something
51. Oppressive - marked by unjust severity or arbitrary behavior
52. Opprobrium - state of disgrace resulting from public abuse
53. Optimist - a person disposed to take a favorable view of things
54. Optional - possible but not necessary; left to personal choice
