Opera add-ons are not just for Firefox. There are several good ones for Opera as well. They can be used on Windows, Linux and Mac OS X.
Twitter Opera Widget
Twitter Opera Widget is the most popular Twitter plugin for Opera. It has five tabs: Recent (the normal timeline), Mentions, Direct, Archive (your profile), Near (which shows tweets from your geographic area) and Search. Of course you can also post new tweets. Twitter Opera Widget can also be set to autorefresh and to be always on top of other windows.
Twitter Opera Widget can be downloaded from the Opera Widget site. When you press the Launch button, the widget is started and Opera presents a window asking whether you want to keep the widget. You can first try out the widget and then press yes or now. If you keep the widget, it will be accessible from the Widgets menu in Opera and can later be removed from Manage widgets.