Your post is a bit cryptic, so I'm not sure, but on the face of it, it looks to me like errors in your html, rather than a bug in opera. What are you trying to do with the src="about:blank" attrib? What do you mean by "deined:"?
Still not sure what you want to do by saying 'src="about:blank" '. Do you have an html file called blank.html? If yes, then use 'src="blank.html" '. If not, and you are trying to dynamically create the pop-up, just remove the 'src="about:blank" '. The bug isn't with Opera; Opera is just less forgiving of errors.
The need for a timeout seems strange... Onload() should only fire once the frameset has completed loading all documents in its frames. It seems it is firing prematurely.
As far as I know, src="about:blank" is illegal on any and all browsers! Perhaps it validates (haven't tried), but its not valid! Leaving out the src attribute is perfectly valid html, and nothing will be loaded in the frame.