Databases
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OATD
OATD.org aims to be the best possible resource for finding open access graduate theses and dissertations published around the world. Metadata (information about the theses) comes from over 1100 colleges, universities, and research institutions. OATD currently indexes over 4,672,910 theses and dissertations.
OECD Economic Surveys
The Organization for Economic Cooperation and Development publishes an Economic Survey every two years for Fourty countries, including the BRIICS.  It identifies the main economic challenges faced by the country and analyses policy options to meet them.
OED Online
Full text of the 20 volumes of the OED, the 3 volume Additions Series, as well as at least 1,000 new and revised entries added quarterly. The OED is the leading authority on the evolution of the English language over the last millenium, providing information on the meaning, history, and pronunciation of over half a million words.
Open Access Theses and Dissertations
OATD.org aims to be the best possible resource for finding open access graduate theses and dissertations published around the world. Metadata (information about the theses) comes from over 1100 colleges, universities, and research institutions. OATD currently indexes over 4,672,910 theses and dissertations.
Open Directory of Open Access Repositories
OpenDOAR is the quality-assured global directory of academic open access repositories. It enables the identification, browsing and search for repositories, based on a range of features, such as location, software or type of material held. Tools and support enable both repository administrators and service providers to share best practice and improve the quality of the repository infrastructure.
Open Dissertations
OpenDissertations is an open-access database built to assist researchers in locating both historic and contemporary dissertations and theses.
Open Library of Humanities
The Open Library of Humanities is a non-profit open access publisher for the humanities and social sciences. It includes titles such as 19: Interdisciplinary Studies in the Long Nineteenth Century, Comics Grid: The Journal of Comics Scholarship and The Theoretical Roman Archaeology Journal.
OpenDOAR
OpenDOAR is the quality-assured global directory of academic open access repositories. It enables the identification, browsing and search for repositories, based on a range of features, such as location, software or type of material held. Tools and support enable both repository administrators and service providers to share best practice and improve the quality of the repository infrastructure.
Oxford Art Online
Encyclopedic coverage of art, archaelogy, architecture, design and film. Electronic version of the Concise Oxford Dictionary of Art Terms, the Encyclopedia of Aesthetics, the Grove Dictionary of Art, and the Oxford Companion to Western Art
Oxford Bibliographies Online: Classics
Offers peer-reviewed annotated bibliographies on specific topics in a specific subject area. Includes guides to introductory works, textbooks, guidebooks, journals, reference works, etc., and links to useful websites. Contains a "My OBO" function that allows users to create personalized bibliographies of individual citations from different bibliographies.
Oxford Bibliographies Online: Medieval Studies
Offers peer-reviewed annotated bibliographies on specific topics in a specific subject area. Includes guides to introductory works, textbooks, guidebooks, journals, reference works, etc., and links to useful websites. Contains a "My OBO" function that allows users to create personalized bibliographies of individual citations from different bibliographies.
Oxford Classical Dictionary
The electronic version of the 3th ed. Covers all aspects of the Greco-Roman world, from politics and economics to religion and philosophy to literature and art to social history and gender.
Oxford Encyclopedia of Ancient Greece and Rome
Offers a comprehensive overview of the major cultures of the classical Mediterranean world—Greek, Hellenistic, and Roman—from the Bronze Age to the fifth century CE. It also covers the legacy of the classical world and its interpretation and influence in subsequent centuries.
Oxford Encyclopedia of the Music of India
Traces the story of Indian music through 2,000 years of history. Entries cover forms of music from classical to film-originated, as well as dance styles, technical terms, instruments, and biographies of vocalists, musicologists, saint poets, gurus, composers, and instrumentalists. The work spans not only all regions of India, but also covers the music of the Subcontinent, including Nepal, Bangladesh, Pakistan, and Sri Lanka.
Oxford English Dictionary
Full text of the 20 volumes of the OED, the 3 volume Additions Series, as well as at least 1,000 new and revised entries added quarterly. The OED is the leading authority on the evolution of the English language over the last millenium, providing information on the meaning, history, and pronunciation of over half a million words.
Oxford Music Online
Encyclopedic coverage of musical subjects of all kinds including composers, performers, conductors, musical terms and forms, individual works, instruments and notations, venues, and genres. Electronic version of the New Grove Dictionary of Jazz, the New Grove Dictionary of Music and Musicians, the New Grove Dictionary of Opera, the Oxford Companion to Music, and the Oxford Dictionary of Music. Cross-searchable with the Encyclopedia of Popular Music
Oxford Text Archive
"The University of Oxford Text Archive develops, collects, catalogues and preserves electronic literary and linguistic resources for use in Higher Education, in research, teaching and learning." This includes texts from antiquity to the modern period ready for text mining and available in XML, HTML, TXT, or ePub formats. 
Note: Some resources are only available to users affiliated with the University of Oxford, but many are freely accessible.