OK now I have "somewhat" found the solution. I tested on a different page, and the dialogue looked exactly like in your example.
But I was specifically asking for the purposes of my company's website, where there is a dynamically-generated product sheet which automatically opens the print dialogue. The sheet is designed for portrait format, so the CSS code of the page contains @page {size: portrait;} and removing that part of the code actually solved the issue. So I guess I have found a bug? Because Opera does actually interpret this piece of the code, which is good, but interprets it "too strongly" - as in, it does not only decide the orientation of paper for you, but also the format, margins, etc. and doesn't allow you to change anything