The shortcut is Control–Tab and Control–Shift–Tab.
By enabling the Advanced keyboard shortcut setting, you can use 1 and 2.
In the upcoming Opera 12, you can use Command–Option–Left and Right as well.
You can achieve the specific shortcut you wanted by modifying the Preferences file. This is unsupported and I will thus not provide exact steps. There are other examples in the forums and elsewhere on the Web for doing things like this.