Operas 168,413 Officials and employees 91,474 Orchestral music 57,256 Older people 41,535 Organ music 33,591 Officers 24,868 Overtures 23,963 Opera 17,025 Occupations 17,002 Oratorios 16,339 Orthodox Judaism 15,706 Origin 13,889 Organizational change 12,400 Orthography and spelling 12,074 Oceanography 11,545 Obstetrics 11,159 Occupational training 10,187 Orchestre, Musique d' 10,165 Occultism 10,159 Opéras 9,982 Ontology 9,573 Organized crime 9,543 Orphans 9,023 Outdoor recreation 8,912 Organizational behavior 8,544 Orchestral music, Arranged 8,467 Organizational Behavior 8,237 Organization 7,814 Operating systems (Computers) 7,455 Orthodox Eastern Church 7,279 Old age pensions 7,239 Outlines, syllabi, etc 7,077 Organizational effectiveness 6,148 Oncology 5,583 Observations 5,513 Optics 5,488 Oratory 5,459 Ordinances, Municipal 5,393 Organ (Musical instrument) 5,374 Ophthalmology 5,373 Operas, Chinese 5,122 Oral tradition 4,999 Organizations & Institutions 4,955 Orgue, Musique d' 4,931 Oral history 4,862 Online social networks 4,797 Organic compounds 4,626 Olympics 4,394 Operations research 4,368 Obligations (Law) 4,343 Orthopedics 4,329 Obesity 4,067 Ore deposits 4,039 Old age 3,955 Optical data processing 3,947 Oil fields 3,849 Office 3,789 Ouvertures (Orchestre) 3,757 Oil and gas leases 3,663 Oral communication 3,648 Operational readiness 3,518 Office buildings 3,461 Ordination sermons 3,455 Occupational diseases 3,447 Ojibwa Indians 3,411 Orchids 3,296 Organ music, Arranged 3,268 Older women 3,228 Outlaws 3,221 Oaths 3,212 Overland journeys to the Pacific 3,168 Optical properties 3,092 Orphanages 3,059 Oil spills 3,031 Optoelectronic devices 3,008 Ocean 2,949 Old English 2,889 Objectivity 2,815 Optical materials 2,733 Object-oriented programming (Computer science) 2,712 Opinion publique 2,680 Old age assistance 2,614 Optical instruments 2,613 Ordnance 2,537 Opposition (Political science) 2,479 Optical communications 2,467 Organizational sociology 2,411 Otolaryngology 2,393 Organizational Innovation 2,334 Other 2,288 Ocean travel 2,273 Open source software 2,272 Organizational learning 2,265 Ordinateurs 2,195 Old Norse literature 2,166 Oxygen 2,163 Occupied territories 2,138 Organic farming 2,120 Occupational Diseases 2,078 Ornithology 2,060