This isn't the "official" solution, but a quick note on how I jury rigged my logging to get it working.
Both the OES WebLogic SM and the Enterprise Manager seem to use log4j to do their logging. Since the SM spins up first you will see the normal sorts of things you'd expect in the OES SM's log file as the SM WebLogic server boots. Then suddenly the log juts stops.
This is because the Enterprise Manager app (EM) re-initializes the log4j system with its own config file. To solve it just add the lines you want from OES' log4j.properties to the config file ./config/fmwconfig/servers/AdminServer/applications/em/META-INF/emomslogging.properties
Hope this helps someone else!