OC4J does not like the funky server-side redirect that Roller uses to return the logging-in user to the page where they clicked the login link. This problem was causing a recursive redirect to redirect to redirect to infinity. Mozilla recognized the problem and put up that "Redirection Limit" popup, which is nice. IE did not recognize the problem and caused OC4J to crash, which is, to say the least, not nice.
This was an easy fix for me. I just switched my login-redirect.jsp page to use a client-side redirect rather than a server-side redirect and everything is peachy.
The next problem is XML related. Roller can't seem to read its XML menu definition file. The code, which works fine on Resin and Tomcat, uses ServletContext's getResourceAsStream() to get the XML file. On OC4J, the XML parser (called by the Jakarta Digester) blows up with a "XML-0220: (Fatal Error) Invalid InputSource" error. Rats! Time for some more googling.
UPDATE: this turned out to be a missing slash in the path to the menu config file under WEB-INF. I added the slash and that fixed Roller on OC4J and did not break Roller on Tomcat. Now Roller works perfectly on IE, but can't find it's stylesheets on Mozilla. I'll save this last problem for tomorrow.