That’s why Opera can be set to ignore core X fonts, by unchecking the Enable Core X fonts configuration option. You can use this as a workaround.
So, the result of having this option enabled can be compared in the following images. The first image shows the rendering when the
font-family: "Deja Vu Sans", sans-serif directive is used. The second image shows the same page when the «Deja Vu Sans» option is removed from the directive. In this second case, Opera renders the text using the «sans-serif» font, whatever it matches.
Seen from a different perpective, it appears to be that when Opera asks X for a font, X returns an antialised core font. Does this happen on other distributions too?