No solution to offer but I concur with conticreative's post.. Opera's spell check leaves much to be desired.
There are a number of words that Opera will identify as "wrong" but are indeed spelled correctly..
Ad Hominem for example. I get that is 2 words.. but 'Hominem' is spelled correctly. I have tried "add to dictionary" a number of times.. and even as I type this out - Opera is telling me it's spelled wrong. There are a ton of examples of this.. It's infuriating.. Spelling is NOT my strong suite, so when I try to use Opera's suggestions, frequently they are wrong and I end up copy/pasting the word in question into a search engine (or highlighting the word, Right Click and Search).. which never fails to provide the correct spelling.
New to these forums, but if I could suggest a fix for a future release - PLEASE fix this!! or at least allow users to import their own "dictionaries" and make sure Opera correctly reference it.