This is a javascript that sets all youtube videos to high quality. To use it with Opera first download and extract it somewhere on your computer. Then in Opera go to youtube.com and right click on any blank white space then select "Edit site preferences..." from the right click menu. Next click the "Scripting" tab and select "Choose" at the bottom to browse for the folder you just unzipped the script to. Now whenever you play a youtube video it will use the high quality settings which look makes the video much crisper and uses stereo audio. Download youtube high quality javascript.
For an example of the difference using the high quality setting makes check out this video in standard quality versus it in high quality. If you have the high quality script already enabled it will automatically enable high quality so you will not be able to compare them.