Monopolies don't come into it
the reason for antitrust with Microsoft was because IE was initally based on Spyglass Mosaic and due to some licence shenanigans they weren't allowed to sell it. Bundling it into a sold product (Win 98) invalidated whatever licence they had, well that's the argument. As such IE is not part of windows, it's an add on and they can't bundle it over and above other browsers, hence the windows without IE option in the EU.
Google on the other hand didn't borrow the source code for their browser so they can do whatever they like, and if chrome OS is essentially chrome with a lightweight hardware interface i don't see where opera would fit in. The vision is to have apps on the web, so how would you even install opera...
If you really want a lightwieght opera browser OS, just bare bones some linux and put it on. Don't expect google to do it for you.