if you want some help in debugging this, please can you be more precise, which version of Wordpress do you use and which version of opera?
If I understand you correct, you are writing a new entry in your blog and see this behavior. I doubt it is a theme specific issue, as the admin pages are not influenced by the them (usually).
P.S.: By the way, the friends on a Wordpress forum, the Wordpress mailing list, or IRC would most likely be very interested in helping you solve this issue. They have the most interest, that Wordpress runs well cross browser.
Most recent WP most recent Opera, no betas. I am well aware that the wordpress forum can probably help, though I thought the members at sitepoint would be a good first call to see if it's something stupid i've overlooked
It would be my guess that if it happens in Opera and not other browsers, it's a browser issue and not a WP problem.
It could be a problem with the one of the files in WP, but according to the newest install, it validates strict. Of course we know from experience with IE and css that validation isn't always the key to solving problems.
It does seem odd that it would extend instead of just truncate.
I recently had a terrible problem with WP. It's another issue, but what I found was that it was a plugin that wasn't playing nice. One check you could do is to deactivate your plugins and see what happens then.