OII may refer to:

• OII, market exchange symbol for Oceaneering International
• Office of Innovation and Improvement, an office of the United States Department of Education
• Organisation Intersex International, a global advocacy and support group for people with intersex traits
• Oxford Internet Institute, a multi-disciplinary department, part of the University of Oxford, England

• Organisation Intersex International Australia
• OII Europe
• Oii-Chinese, the Chinese language affiliate of Organisation Intersex International

This disambiguation page lists articles associated with the title OII.
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