I found you something definitive. From www.xom.nu:
Java 1.3 and earlier do not have a built-in XML parser so in these environments you'll also need to install XOM's supporting libraries. These include xalan.jar, xercesImpl.jar, normalizer.jar, and xml-apis.jar, and are found in the lib directory. The versions shipped with XOM are quite a bit faster and less buggy than the ones bundled with the JDK, so you may well want to use them even in Java 1.4 and later. For example,
$ java -classpath xom-samples.jar:xom-1.1.jar:lib/xml-apis.jar:lib/xercesImpl.jar:lib/normalizer.jar:lib/xalan.jar nu.xom.samples.PrettyPrinter filename.xml
You can leave out xalan.jar if you don't use any of the classes in nu.xom.xslt. normalizer.jar is needed in all versions of Java. However, it's only actually used by the setUnicodeNormalizationFormC() method in Serializer. If you don't call this method, you can omit this archive in space-limited environments. junit.jar is only used for testing, and is not needed for normal operation of XOM.
So leave off xalan.jar. Android's native javax.xml.transform does all that work already (as of API 8).
Incidentally, looks like you may be able to drop some other jars as well.