What is an OPML file?
An .OPML file is an Outline Processor Markup Language File. It uses a standard XML format to structure and exchange outline data between different software applications. Originally developed by UserLand Software, it is now the standard way to export and import RSS feed subscription lists. You will often encounter these files when transferring feeds between RSS readers or downloading channel subscriptions from Google YouTube.
How to open OPML files?
Many different applications use .OPML files. RSS readers like NetNewsWire and FeedDemon use them to import and export subscription lists. Modern note-taking apps and outline processors like Dynalist and software from The Omni Group (such as OmniOutliner) also rely on this format to save document structures. Because they are plain XML or UTF-8 text files, you can open them in any basic text editor, but the raw markup code can be difficult to read.
Best practices and troubleshooting
Because an .OPML file can contain different types of data—ranging from simple text notes to complex web feed directories—it is not always obvious which application created it. viewer.online/opml analyzes .OPML files to identify their exact format and creator software, shows which programs can open the file, and usually previews it. If you need to transform your outline data into standard formats like .CSV, .JSON, or .TXT, we recommend using convert.guru for fast file conversion.
Summary
viewer.online/opml directly opens and previews .OPML files in the browser, eliminating the need to install software or troubleshoot compatibility issues.