as far as java goes for opera, it's not like setting it up like you would with mozilla. open opera and go to File > Preferences > Multimedia. make sure the Enable Java button is ticked. also underneath that, there's a text box. this is where you tell opera the java path. depending on your version and java package (jre vs. sdk), the path will look something like this:
that's mine using the sdk from sun version 1.4.2_01. then you can click the validate path button. if the path validates. close opera and reopen it.
then you can go here to see if java works.
as far as the mplayer plugin goes, that doesn't work with opera. in opera for linux, most everything needs to be done in the File > Preferences > Filetypes. usually stuff for like quicktime, etc... you'll probably want to specify the application instead of using the plugin.