how to fix the Error:opening/initializing the selected video_out (-vo)device

you are getting this: Error opening/initializing the selected video_out (-vo)device error when you run mplayer , open terminal and run this command:
code:

 gmplayer -vo x11 -ao oss

and play your file. REMEMBER this only applies after installing mplayer and codecs.

I find many ubuntuers besides me meet this problem,hope this can help you.
Another question is mplayer can't play a movie file by fullcreen, then you should edit ~/.mplayer/config and just add the line:

Code:

zoom=yes

that seemed to do the trick .