You can copy-paste the contents of an OPML document directly into any WorkFlowy item, and it’ll be imported as a big nested list.
Our export dialog includes an OPML option. To get to the export dialog, you just hover over the bullet point of any item (or click on the 3-dot menu to the left of any bullet), or if you want to export everything at once, the menu at the top right of the page lets you do so.
Please note, other outliner apps may require you to create an OPML "file". In this case, simply choose the OPML export option and then click on the "download" link at the bottom of the popup.