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| • | OAA - ORGANISMO ARGENTINO DE ACREDITACION | |
| • | OAISTER | |
| • | OCDE / OECD - ORGANISATION FOR ECONOMIC COOPERATION AND DEVELOPMENT | |
| • | OCLA - OBSERVATORIO DE LA CADENA LACTEA ARGENTINA | |
| • | ODLIS: ONLINE DICTIONARY FOR LIBRARY AND INFORMATION SCIENCE | |
| • | OEA - ORGANIZACION DE LOS ESTADOS AMERICANOS | |
| • | OIML - ORGANISATION INTERNATIONALE DE METROLOGIE LEGALE | |
| • | OIT - ORGANIZACION INTERNACIONAL DEL TRABAJO | |
| • | OMNEXUS | |
| • | OMPI - ORGANIZACION MUNDIAL DE LA PROPIEDAD INTELECTUAL | |
| • | OMS - ORGANIZACION MUNDIAL DE LA SALUD | |
| • | ONELOOK DICTIONARY SEARCH | |
| • | ONLINE BOOKS PAGE | |
| • | ONN (NC) - OFICINA NACIONAL DE NORMALIZACION (CUBA) | |
| • | ONU - ORGANIZACION DE LAS NACIONES UNIDAS | |
| • | ONUDI - UNIDO - UNITED NATIONS INDUSTRIAL DEVELOPMENT ORGANIZATION | |
| • | OPEN TEXTBOOK LIBRARY | |
| • | OPENDOAR - DIRECTORY OF OPEN ACCESS REPOSITORIES | |
| • | OPENSTREETMAP - OSM | |
| • | OPENTHESIS | |
| • | ORGANIC SYNTHESES | |
| • | ORGANISMO PROVINCIAL PARA EL DESARROLLO SOSTENIBLE (PROV. BUENOS AIRES) | |
| • | OSHA - OCCUPATIONAL SAFETY & HEALTH ADMINISTRATION | |
| • | OSTI.GOV - OFFICE OF SCIENTIFIC AND TECHNICAL INFORMATION, U.S. DEPARTMENT OF ENERGY | |
| • | OUTSOURCINGPHARMA | |
| • | OXFORD DICTIONARIES | |
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