Officials and employees 4,191 Operas 2,273 Outdoor recreation 1,040 Ore deposits 905 Organizational change 558 Outer space Exploration 504 Operational readiness 471 Oceanography 447 Occupational training 428 Organizational behavior 424 Older people 414 Operations research 410 Occupations 390 Older people Services for 364 Operating systems (Computers) 357 Oil and gas leases 335 Oil fields 311 Organizational effectiveness 291 Overland journeys to the Pacific 262 Older people Medical care 252 Organization 250 Oil-shales 248 Online social networks 239 Organ music 227 Older people Health and hygiene 221 Optical data processing 214 Open source software 212 Oncology 210 Ore-dressing 203 Organic compounds Synthesis 203 Organized crime 180 Obesity 179 Object-oriented programming (Computer science) 179 Operator theory 174 Optics 173 Older people Care 160 Ocean bottom 159 Oregon 145 Opioid abuse 143 Ontology 142 Optical pattern recognition 141 Organic compounds 138 Orchestral music 137 Outcome assessment (Medical care) 135 Occupational therapy 133 Older people Long-term care 132 Oil spills 130 Opera 128 Oil spills Environmental aspects 124 Ophthalmology 124 Orthopedics 122 Older people Legal status, laws, etc 117 Opioid abuse Prevention 117 Orphans 115 Optical communications 111 Officers 109 Outer space 99 Orthopedic surgery 97 Organometallic chemistry 96 Organic water pollutants 95 Oratorios 94 Occupational training Law and legislation 93 Older people Employment 93 Organometallic compounds 92 Organizational change Management 89 Organizational learning 88 Oil reservoir engineering 86 Oral communication 86 Older people Economic conditions 84 Oil-shale industry Environmental aspects 83 Organic farming 83 Ocean temperature 82 Office management 81 Optoelectronic devices 81 Orchestral music, Arranged 81 Off-road vehicle trails 80 Occupational diseases 79 Ocean-atmosphere interaction 77 Optical materials 77 Oil wells 75 Otolaryngology 74 Outlaws 73 Oxidative stress 73 Office practice 72 Older people Housing 72 Older people Medical care Law and legislation 72 Oklahoma 71 Older people Social conditions 71 Older people Abuse of 70 Organic compounds Environmental aspects 69 Opioid abuse Treatment 68 Oxidation 68 Older people Abuse of Prevention 67 Ores Sampling and estimation 67 Observation (Educational method) 66 Obstetrics 66 Older people Government policy 66 Ostracoda, Fossil 66 Osteoporosis 65 Oil and gas leases Environmental aspects 64