Dustin Wilson, also known as Khadgar in the MyOpera community has been able to tweet by configuring Opera.
First you have to configure it by adding the data URI:
To use it follow these directions:
1. Copy the long data URI into a text editor and do a text replacement for “opera” then replace it with your twitter username.
2. Do a text replacement for “software” and replace it with your password.
3. Go to Tools → Preferences (or Opera → Preferences on Mac OS X).
4. Click on the Search tab then click the Add… button.
5. In the input box labeled Name type in Twitter.
6. Type in a suitable keyword in the Keyword field. For example tw.
7. Paste in the data URI from your text editor into the Address field and click OK.
To make it work, you have to login normally to twitter and you are ready to go.