I don't know because I don't use Opera but I would imagine it checks your spelling when you select 'Spellcheck' after you have finished composing text. Perhaps an Opera user can give you the full facts.
From the original link ....
Quote: Using the spell checker With a spell checker installed, the "Check spelling" option will appear in the context menu of all edit fields. When composing e-mail messages, the "Check spelling" button will be visible on the toolbar. To check the spelling of a single word or a portion of your text exclusively, select the text before checking.