What are OPML Files, and How Do You Open, Edit, and Use Them?

Have you ever wanted to easily exchange and organize outlines or list data between apps? OPML files make it possible!

Let me walk you through everything you need to know about Outline Processor Markup Language (OPML) files. We‘ll explore what they are, a bit of history, how you can work with them, and why they provide a simple yet powerful way to structure your content.

Overview – OPML Explained through Examples

Before we get more technical, let‘s build intuition by looking at some OPML file examples and what they allow us to do:

Application 1) Easily export your RSS feed reading list from an app like Feedly and import it into another one like Inoreader – this enables you to seamlessly switch between apps without losing your subscriptions.

Application 2) Effortlessly transfer your workout routine outline from your notes app into a fitness tracking app like Strava. This way you retain structure while moving data.

Application 3) Smoothly migrate a nested sitemap for your website across CMS platforms. Here OPML preserves the hierarchy and relationships across systems.

As you can see, OPML shines when exporting and importing structural content across apps – it‘s a flexible transport format for outlining information by encoding it in lightweight XML.

Now let‘s get into more specifics…

History and Origins

OPML was created by Dave Winer, pioneer software developer and founder of UserLand Software tools. The first version was released in the year 2000.

As Dave recounts, the motivation was to:

"Create an open format to encourage other outlining vendors and developers to participate and share information with each other."

The key goal was interoperability – enabling exchange of outline data between otherwise closed software ecosystems.

This trait gave OPML files increased utility right from their inception. UserLand‘s own Radio Userland app added native support for OPML import/export as early as 2001.

Over the next decade, many outliners and RSS readers integrated OPML capabilities making it a standardized outline exchange format across the industry.

Technical Breakdown – Format and Structure

As the name suggests, OPML is an XML-based markup language focused on encoding outline structures.

The technical specification for OPML 2.0 defines rules and elements for representing hierarchical trees of information.

Here are the key components that make up an OPML file:

Root Element: Encloses the entire document
- <opml>
Head Element: Contains outline metadata
- <head>
Body Element: Holds the actual outline
- <body>
Outline Element: Represents each line item
- <outline>

Additional metadata like OPML generator version may be specified as XML attributes.

Here‘s a snippet view of an OPML file containing a nested outline:

<?xml version="1.0" encoding="UTF-8"?>
<opml version="2.0">

  <head>
   <title>Book Outline</title>
  </head>

  <body>
   <outline text="Introduction">
     <outline text="Preface"/>
     <outline text="Key Terminology"/>
   </outline>

   <outline text="Main Content">
     <outline text="Chapter 1"/>
     <outline text="Chapter 2"/> 
     <outline text="Chapter 3"/>
     <outline text="Chapter 4"/>
   </outline>

   <outline text="Conclusion"/>
   <outline text="References"/>
  </body>

</opml>

When opened in a supported app, an outline renderer transforms this into an interactive hierarchical tree.

Real-world Usage and Applications

Let‘s now explore some common applications and software integrations that utilize OPML capabilities:

RSS Feed Management

One of the most popular uses of OPML files is to import/export RSS feed reading lists and subscriptions between feed reader apps and websites.

For instance, apps like Feedly and Inoreader support seamless OPML data migration flows. You simply export an OPML file containing your feed reading list from one app, and import it into the other.

This makes switching between RSS aggregators super convenient without having to manually setup your feeds again!

OPML streamlines feed management across RSS readers

Outline Sharing

Many outliners like OmniOutliner (MacOS), TreeDBNotes (Windows) also leverage OPML import/export to share and collaborate on outlines.

You can distribute an OPML file to team members working on documenting a process. They can then import this into their preferred outlining app for enhancement keeping the overall structure intact.

Data Migration

Since OPML maintains relationships and hierarchy, developers use it when migrating nested data between programs even beyond traditional outliners.

For instance, bookmark managers like Raindrop.io support OPML export/import to help transfer bookmark collections to other platforms while retaining folder groups and structure.

Cross-platform Support

Being based on portable XML, OPML files can integrate easily across macOS, Windows, Linux, iOS and Android apps.

Any outliner or IDE that builds in an OPML parser can enable cross-platform exchange.

For example, using Github‘s online OPML renderer at http://opml.github.io allows you to visually check OPML files on any device with a modern web browser!

Working with OPML Files

While we‘ve discussed several use cases above, you might be wondering – how do I actually view or edit an OPML file on my own device?

Well, it‘s quite straightforward…

Opening and Viewing

To simply open and review the contents of OPML files you have a couple of options:

Any text editor: Since it‘s essentially XML, you can use text editors like Notepad++, VSCode, Sublime etc to open and view the markup
Online OPML renderer: Quickly visualize the outline structure using web tools like http://opml.github.io
Outliner apps: Use an outlining program like OmniOutliner which treats the OPML as a native file format

OPML displayed in a text editor vs outline viewer app

Editing and Modifying

If you want to edit an OPML file to tweak the structure or content, open it in one of the following programs:

XML Editor: Modify the markup in a standalone XML editor like XML Marker
Outline Apps: Edit it visually by importing into an outliner tool and then export back out
Simple text editor: Quick changes can be done in any basic text editor

After editing, double check the updated OPML file renders properly using an online OPML viewer.

Converting and Exporting

To export or convert OPML files into alternate formats, you can take several approaches:

HTML Export: In an OPML-compliant app, save as HTML to transform into a webpage document
PDF Export: Print the OPML outliner to a PDF layout retaining structure
CSV Export: Use online converter tools like https://opml2csv.com to get a spreadsheet format
Markdown Export: Convert OPML hierarchies into Markdown for further editing and portability

This enables you to unlock an OPML file in different productivity tools for further processing and analysis.

Advance Usage Tips

Beyond the basics, power users can further enhance their OPML capabilities in these ways:

Store code GitHub Gists as <outline text> elements
Embed media via <outline text="alt" type="rss" xmlUrl="url">
Add custom XML attributes for proprietary metadata
Develop a custom OPML editor using toolkit libraries for Node.js etc

As Dave Winer puts it, "OPML is very simple, but also infinitely extensible".

The built-in support for recursion allows you to model complex relationships as per your specific needs.

Comparative Analysis

You might be wondering how does OPML stack up against other data serialization formats? Let‘s evaluate:

	OPML	JSON	CSV
Structured outlines	âœ”ï ̧ Yes	Partial	No
Human readable	âœ”ï ̧ Yes	Partial	Barely
Compact size	âœ”ï ̧ Yes	âœ”ï ̧ Yes	âœ”ï ̧ Yes
Data validation	âœ”ï ̧ Yes	âœ”ï ̧ Yes	No
Metadata support	âœ”ï ̧ Yes	No	No
Tooling ecosystem	Medium	Large	Large

JSON: More concise syntax but no native outline modeling
CSV: Tabular format making outlines difficult
YAML: Space sensitiveindentation for structure

As shown above, OPML provides the best combination of hierarchy modeling, readability and portability.

The Evolution of OPML

Let‘s briefly discuss the release history:

OPML 1.0

The original spec developed by UserLand in 2000 formed the basis. Available serialization formats at the time imposed limitations which OPML v1.0 tried solving. Some key goals achieved:

Simplicity
Extendable to new data types
Editable in plain text editors

It received widespread voluntary adoption and demonstrated the need for such a format.

OPML 2.0

In 2005, a public draft sought feedback on ideas for enhancing OPML further. Based on inputs, v2.0 released in 2006 with following improvements:

Better XML structure for validation
Explicit encoding parameter
Multi-level outlines supported
Independence from other standards

This modernized specification paved the way for growth.

The Future

V2.0 continues to be the current standard for OPML as of 2023. The original inventor Dave Winer along with other collaborators maintain oversight through opml.org.

Recent developments like the OPML editor have increased accessibility. Some feel the language could better leverage newer conventions like JSON or YAML. The maintainers however believe extending via profiles is a better evolutionary path for OPML.