OPML is an acronym for Outline Processor Markup Language. The OPML file format is used to exchange information between applications that run on different operating systems. The OPML format is based on an XML format developed for outlines.
The OPML file format was initially created by Radio Userland. It was developed to be the native file format for outliner applications. However, since its inception other programs have adapted the file format for other uses.
OPML files are commonly used to to exchange RSS subscription lists between different feed aggregators.
The original OPML file format has now been replaced by the more versatile OPML 2.0 file format.
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If you have additional information about the OPML file format or software that uses files with the OPML suffix, please do get in touch - we would love hearing from you.