Note that there are several things that have to be done to completely install and configure J2SE 5 under Tiger. To simplify things, I've written up a quick FAQ on how I did this.
I know this isn't specifically an oXygen issue, but some people are unable to run the plugin for Eclipse M7, which is working fine for me. SO I thought this might help.
Having trouble installing <oXygen/>? Got a bug to report? Post it all here.