Thanks for the details, Sorin.
By default the html extension is not associated with oXygen in this way because generally HTML is not XML.
This makes good sense. What bothers me, though, is that oXygen doesn't think it can open text (.txt or no extension), Perl (.pl, .pm, .perl), shell-script (.sh, .bash) or lots of other files I know it can open. I'm guessing this is because the data in the Info.plist file also associates an icon, and perhaps a default application, with an extension, and y'all did not want to be so presumptive as to suggest that everyone would want their Perl programs, etc., to have an oXygen icon or open in oXygen by default. This also makes good sense. But I'm wondering -- is there any way to tell Mac OS X that it is OK to let oXygen open a file, even though it is not the default and does not have an oXygen icon? (Or have I completely misunderstood this stuff?) If I had my druthers, oXygen would only associate its icon and be the default application for .xsl, .fo, .xslfo, and .xslt files, but would be *able* to open almost anything that I explicitly drag on top of it or try to open via the Open dialog box. I am sure that other people have similar desires with different lists of file types. Thanks again.