This page lists the types of file that Opal can open. They are:
Native Opal outline files. A native file has extension .otln.
Acta files. Acta was Opal’s predecessor in the world of System 6, System 7, Mac OS 8, and Mac OS 9 (and Classic), and is now available free. An Acta document has type code 'otln' or 'OTLN'. Opal can open an Acta file directly, with preservation of all contents.
Text files. A text file has extension .txt or type code 'TEXT'. Tab characters at the start of a line will be taken as outline hierarchy indicators, and will be stripped. (But Opal will open any text file, even if its line-initial tab characters are incoherent as outline hierarchy markers.) Acceptable text encodings are UTF-8 and UTF-16, MacRoman, and WinLatin-1; if Opal can’t guess the encoding, it puts up a dialog asking which it is.
So, for example, this text file:
Types of vorble [TAB]Left-handed [TAB]Right-handed [TAB][TAB]Right-handed vorbles are rare, though Types of dingbird [TAB]There is no such thing as a dingbird
...will open as an Opal file like this:
OPML files. OPML is a crude XML format describing a plain text outline. An OPML file has extension .opml. A number of popular outliner applications use OPML as a medium of exchange. Opal treats OPML files as OPML 1.0; additional information permitted under OPML 1.1 or OPML 2.0 is ignored.
An Acta file, text file, or OPML file is effectively opened by importing. To save the imported file as an Opal document, choose File > Save As.