OPML

• OPML

• 

  
  Here’s a breakdown of what OPML (Outline Processor Markup Language) is, how it works, and where it’s used:
  
  

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• What it is

• OPML is an XML-based file format designed to represent outlines — hierarchical trees of nodes with attributes. (Wikipedia)
• The root element is <opml version="x.y">, containing a <head> (metadata) and a <body> (the outline content). (The Library of Congress)
• Each node in the outline is represented by an <outline> element, which can have attributes like text, type, xmlUrl, etc., and may contain nested <outline> children. (OPML)

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• Key characteristics

• Designed to be human-readable (since XML) and easy to export/import between systems. (Listparser)
• Flexible: while originally intended for outliner applications, it became heavily used for feed subscriptions (RSS/Atom) because the structure fits lists of feeds very well. (Marshall Kirkpatrick)
• Because it uses XML and tree structure, it's portable across operating systems and applications. (file.org)

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• Common uses

• RSS feed aggregators: Exporting or importing user’s subscription lists in OPML is common. For example, you subscribe to many blogs or podcasts; exporting as OPML allows you to move to another reader easily. (Lifewire)
• Outliner/Note apps / Mind-map tools: Because OPML supports hierarchical outlines, many note-taking or outliner tools support import/export in OPML. (Literature & Latte)
• Knowledge/PKM sharing: In contexts like PKM and Logseq, OPML could potentially serve as an interchange format between tools that represent outlines, theoretically speaking, hierarchies, etc, but I've never seen this before.

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• Structure example (simplified)

  <opml version="2.0">
  <head>
    <title>My Feeds</title>
    <dateCreated>2025-11-08T12:34:56Z</dateCreated>
    <ownerName>M</ownerName>
  </head>
  <body>
    <outline text="Tech Blogs">
      <outline text="AI & ML" type="rss"
               xmlUrl="https://example.com/ai-feed.xml"
               htmlUrl="https://example.com/ai-blog"/>
      <outline text="Distributed Systems" type="rss"
               xmlUrl="https://example.com/ds-feed.xml"
               htmlUrl="https://example.com/ds-blog"/>
    </outline>
    <outline text="Podcasts" type="rss"
             xmlUrl="https://podcast.example.com/feed.xml"
             htmlUrl="https://podcast.example.com"/>
  </body>
  </opml>

  
  This illustrates how you can nest categories and individual feed entries.