As tax laws differ in every country, I agree with Valeria - you should find a tax advisor in Italy. I would guess it's up to you to declare the extra income - oDesk produce a few reports for this purpose, although at the moment I think you can only generate a certificate of income for the 365 previous days. This means (at least in my (Irish) case, if you need to declare for the calendar year, you have to go to your computer on Dec 31 to produce and print the report for the year.
Bear in mind though that this is only a statement of income to you from oDesk - they have no part in the taxation process (unless you're an American citizen). This is only my Irish vierw on it - presumably you'll get some responses from other Europeans that may be more helpful.
