Opal outline file
File type: Various data file
An .otln file is an outline document created by Opal, an outlining and organization application for Apple Mac computers.
It stores information arranged as a hierarchical outline, such as ideas, reports, lists, notes, projects, and graphics.
File type category:
Various data file
Date updated: September 28, 2026
Opal is Mac software for organizing information in an outline. An OTLN file is its outline document and stores the content and structure of an outline.
The format is intended for outlines containing items such as ideas, reports, lists, notes, projects, and graphics. The file can therefore represent both the text of an outline and the relationships between its entries.
OTLN files belong to the Mac version of Opal, originally associated with Mac OS X and available for Apple macOS. They are not general-purpose text files, so their outline structure may not be preserved when opened in another application.
Use Opal on Apple macOS or Mac OS X to open an .otln outline file.
If Opal is not installed, a general text editor may not display the outline correctly because OTLN is an Opal document format.
Open the file in Opal and use its export or copy-and-paste features to move the outline into a format supported by the installed version.
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