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OAPEN promotes and supports the transition to open access for academic books by providing open infrastructure services to stakeholders in scholarly communication. 
OMICS International
OMICS International is an Open Access publisher. It owns and operates 700+ peer-reviewed clinical, medical, life sciences, engineering, and managment journals. 
OmniFile Full Text Mega
Note: Access via Hong Kong Public Libraries
OmniFile Full Text, Mega Edition provides a wide range of research resources. It contains full text of articles from over 3,100 publications dating back to 1994. It also contains abstracts and indexing from nearly 5,100 publications as far back as 1982. The subjects covered include applied sciences, art, education, humanities, law, social sciences, technology, etc.
Online Dictionary for Library and Information Science (ODLIS)
ODLIS is designed as a hypertext reference resource for library and information science professionals, university students and faculty, and users of all types of libraries. 
Open Access Library (OALib)
OALib is currently hosting links and metadata to more than 5,691,949 open access articles covering a wide range of academic disciplines. 
Open Access Theses and Dissertations (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 5,877,305 theses and dissertations.
Open Book Publishers
It is an open access academic book publisher based in the United Kingdom. It promotes open access for full academic monographs, critical editions and textbooks in the Humanities, Social Sciences, Mathematics and Science. All OBP books are peer-reviewed.
Open Textbook Library
The Open Textbook Library is a comprehensive referatory that points to open textbooks by a variety of authors and publishers.
It is a nonprofit organization based out of Rice University and is one of the leading providers of OER textbooks. OpenStax textbooks are available as free downloads in PDF, Web, and EPUB formats. The textbooks are peer-reviewed, and openly licensed.
A search engine for activity ideas using simple items you can find in every household, out of the need to support parents find daily activities they can do to help develop and promote their children's developmental skills. 
OTseeker is a database that contains abstracts of systematic reviews, randomised controlled trials and other resources relevant to occupational therapy interventions. 
Oxford Academic
Oxford Academic, Oxford University Press (OUP)'s platform for research, offers a single point of entry for access to scholarly and academic books and journals. 
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