Just an additional note: It's not enough to just install mmm-mode and psgml according to the INSTALL files that come with them (they are generic). At least I didn't get the needed .el and .elc files copied to /usr/share/emacs/site-lisp, so I had to manually make subdirs for both of them in that dir and then copy the files over.
.emacs file also has to be configured so that they get loaded:
(autoload 'sgml-mode "psgml" "Major mode to edit SGML files." t)
(autoload 'xml-mode "psgml" "Major mode to edit XML files." t)
(require 'mmm-auto)
(add-to-list 'load-path "/usr/share/emacs/site-lisp/mmm")
(add-to-list 'load-path "/usr/share/emacs/site-lisp/psgml")
(add-to-list 'load-path "/usr/share/emacs/site-lisp/oacs")
(require 'oacs)
