Universal Decimal Classification

  1. Object and scope of sociology
  2. Objects in worship. Furnishings and decorations
  3. Observation and recording of phenomena. Visual indication of phenomena
  4. Occupational health hazards. Occupational health and hygiene
  5. Occupations and activities associated with music
  6. Occupations and activities associated with the arts and entertainment
  7. Oceanic Ancestry Group
  8. Oceanic languages
  9. Oceans, seas and interconnections
  10. Offences against fundamental liberties, human rights
  11. Offences against public credit, morality, the family
  12. Offences against the person
  13. Offences against the state
  14. Oils, fats and waxes of animal origin
  15. Oils. Fats. Waxes. Adhesives. Gums. Resins
  16. Old period. Archaic period
  17. Old period. Archaic period
  18. Old persons. Persons in old age. Senior citizens
  19. Oman. Sultanate of Oman. Salṭanat ʻUmān
  20. Omotic languages
  21. One-dimensional. Lines
  22. Online computing systems for specific use
  23. Opening up of territories. Colonization. Colonialism
  24. Operation and control of machines and processes
  25. Operational properties
  26. Operational research (OR)
  27. Operational research (OR): mathematical theories and methods
  28. Ophthalmology. Eye disorders and treatment
  29. Optical apparatus and instruments
  30. Optics
  31. Oratory. Speeches
  32. Order-fixing (double colon sign). Table 1b
  33. Ore and other mineral deposits. Natural resources
  34. Ore deposits (metalliferous mineral deposits) in general. Iron and manganese ores
  35. Ore deposits other than iron and manganese
  36. Ore formation. Mineral formation
  37. Organic chemistry
  38. Organic minerals
  39. Organization of instruction
  40. Organization of science and scientific work
  41. Organization of sport, in general
  42. Organizational behaviour. Management psychology
  43. Organizational management (OM)
  44. Organizational structure of the faith, religion
  45. Organizations and other types of cooperation
  46. Organizations of a general nature
  47. Organizations with humanitarian, philanthropic aims
  48. Orientation. Points of the compass. Relative position
  49. Origin, evolution and domestication of animals kept by humans. Taming of animals. Biological bases of rearing and training
  50. Originator, founder, central figure of the faith
  51. Origins, precursors of scripts. Early forms of scripts
  52. Ornamentals. Decorative plants. Ornamental gardening
  53. Orthodox Judaism
  54. Orthopaedics in general
  55. Other animal contests, e.g. with birds
  56. Other animals kept by humans
  57. Other Australian languages
  58. Other commercial activities
  59. Other compounds of oxyacids
  60. Other green vegetables. Leaf vegetables
  61. Other Indo-Pacific languages
  62. Other kinds of graphic representation of thought
  63. Other kinds of song
  64. Other marine products
  65. Other materials
  66. Other non-serial separates
  67. Other physico-mechanical effects
  68. Other plants with industrial uses
  69. Other precious, remarkable or rare printed works
  70. Other protestant churches
  71. Other pulp fruits
  72. Other regions. Ancient geographical divisions other than those of classical antiquity
  73. Other religious texts
  74. Other terms descriptive of relationships between entities
  75. Other waterway protection and improvement works. Dykes. Coast protection. Land drainage and reclamation
  76. Outdoor games and recreation
  77. Outer clothing. Tailoring
  78. Outlying Islands in the South Indian and South Atlantic Oceans
  79. Oxygen compounds