I'm sorry you did not like the answer, but with Opera's worldwide market share at 1.9% (in the US under 0.6%) and shrinking developers such as Moxiecode, the makers of the TinyMCE editor used here, simply have to prioritize their time and work on compliance from the top down. If they have time left over then they work on the lower end of the market.
Apple's Safari, even though they only have 5.1% share, works well with virtually everything because Apple has spent a lot of time making sure their browser is as close to compliant with all standards as they can. In other words, they knew with a small share developers and web designers were not going to accommodate them, they would have to do the work on their end.
Browser Worldwide Market share as of April 2011
Internet Explorer: 44.5%
I like Opera, but as a web designer I don't spend a lot of time worrying about making my designs compliant with it given its small share. I always check my designs in Opera and if it is something I can fix reasonably easy without messing things up in other browsers I will do it, but if not then Opera is left out.
You might consider, if you are on Windows, getting an offline blog client such as Windows Live Writer or BlogDesk, both free downloads. They will allow you to bypass the wordpress editor and you can publish directly from the blog client without having to go through the wordpress editor. If you are on Mac, there is Ecto which is quite nice (I use it) and although it is not free, at $20 it doesn't break the bank either.
If you are on Linux or some other OS, I've never done any research into blog clients for them so I can't make any recommendations.
This isn't only an issue at WordPress. Everyone uses the TinyMCE editor so you see the same issues in Joomla and Drupal installations as well. There simply aren't any alternative editors out there.