There is only one file type that uses the .OO3 extension:
OmniOutliner 3 File
Data file created by OmniOutliner, a program used for organizing information; can store information using text, checkboxes, pop-up lists, and spreadsheets; used for various purposes, including note taking, tracking expenses, and creating agendas.
OO3 files can be exported to a variety of text and document file formats, including .KEY, .DOCX, .RTF, .TEX, .OPML, and .HTML.
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Updated: August 28, 2016