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- Observer (1791-2003)
- See: Proquest Historical Newspapers: The Guardian (1821-2003) and The Observer (1791-2003)
- Occupational Health and Safety Information Service (OHSIS)
- See: Info4Education
- Oceanic Abstracts
- ProQuest
- OCLC FirstSearch
- See: FirstSearch
- OECD Data
- See: OECD Statistics Databases or ESDS International
- OECD ebooks
- OECD iLibrary
- OECD Statistics Databases
- OECD iLibrary
- OHSIS Document Alert
- See: Info4Education
- OnePetro
- Society of Petroleum Engineers
- OpenGrey
- INIST-CNRS
- OptionMetrics
- See: Wharton Research Data Service - WRDS
- ORBIS
- Bureau van Dijk Electronic Publishing
- Ovid ebooks
- Ovid Online
- Ovid Online
- Ovid Online
- Oxford Art Online
- Oxford University Press
- Oxford Classical Dictionary
- Oxford Reference Online
- Oxford Companion to Music
- See: Oxford Music Online
- Oxford Companion to Western Art
- See: Oxford Art Online
- Oxford Dictionary of Music
- See: Oxford Music Online
- Oxford Dictionary of National Biography
- Oxford University Press
- Oxford Encyclopedia of Reformation
- Oxford Reference Online
- Oxford English Dictionary
- Oxford University Press
- Oxford Islamic Studies Online
- Oxford University Press
- Oxford Language Dictionaries Online
- Oxford University Press
- Oxford Music Online
- Oxford University Press
- Oxford Reference Online
- Oxford University Press
- Oxford Scholarship Online
- Oxford University Press
- Oxford Western Music
- Oxford University Press