Operas 50,188 Officials and employees 32,261 Older people 21,089 Orchestral music 19,359 Organ music 12,319 Overtures 8,349 Organizational change 7,608 Orphans 6,862 Oratorios 6,387 Origin 5,904 Outdoor recreation 5,848 Operating systems (Computers) 5,681 Occupations 5,579 Opera 5,548 Oceanography 5,105 Organizational behavior 5,102 Officers 4,665 Orthography and spelling 4,334 Organizational effectiveness 4,205 Occupational training 4,161 Organization 4,008 Occultism 3,579 Organized crime 3,572 Ontology 3,493 Oncology 3,367 Obstetrics 3,207 Open Content Alliance Digitization Project 3,069 Outlines, syllabi, etc 3,002 Organizational Behavior 2,971 Obesity 2,928 Outlaws 2,802 Oratory 2,743 Organic compounds 2,717 Online social networks 2,708 Oral tradition 2,696 Orchestral music, Arranged 2,644 Oral history 2,622 Operations research 2,457 Oral communication 2,428 Ore deposits 2,422 Operational readiness 2,393 Orthopedics 2,297 Old age pensions 2,287 Object-oriented programming (Computer science) 2,276 Orthodox Eastern Church 2,037 Organizations & Institutions 2,035 Office 2,033 Optics 1,967 Ophthalmology 1,950 Overland journeys to the Pacific 1,869 Old age 1,846 Observations 1,817 Oil and gas leases 1,763 Ojibwa Indians 1,693 Oil spills 1,665 Oil fields 1,663 Olympics 1,611 Onderwijs 1,569 Occupational Therapy 1,550 Organ (Musical instrument) 1,498 Ocean 1,480 Organizational learning 1,473 Ordinances, Municipal 1,436 Older women 1,381 Open source software 1,381 Objectivity 1,342 Optical communications 1,330 Optical properties 1,327 Old age assistance 1,309 Occupational diseases 1,301 Other 1,286 Organ music, Arranged 1,276 Occupational Diseases 1,239 Office practice 1,231 Ordination sermons 1,207 Ordnance 1,201 Occupational therapy 1,188 Overheidsbeleid 1,155 Organisation 1,143 Orphanages 1,122 Osteopathy 1,119 Organic 1,089 O╠łkologie 1,085 Oude Testament 1,069 Ordinateurs 1,067 Old Testament 1,057 Optoelectronic devices 1,057 Organizational sociology 1,054 Opinion publique 1,036 Oxidation 1,031 Outdoor education 1,015 Outdoor life 1,012 Office management 1,000 Options (Finance) 998 Optical pattern recognition 963 Organic farming 961 Otolaryngology 956 Oxygen 954 Operator theory 924 Old English 918