Opera isn't included in the default repositories of the Ubuntu 10.04 installation. 

Follow these steps and get Opera installed on your Ubuntu computer.

Add the following line to your /etc/apt/sources.list file:
deb deb.opera.com/opera/ stable non-free

Get the Opera GPG key
wget -O - deb.opera.com/archive.key | sudo apt-key add -

Then run:
sudo apt-get install debian-archive-keyring

Update your package list:
sudo apt-get update

Finally, install the Opera web browser:
sudo apt-get install opera

 

You're all set. Opera can be found in the applications menu under Applications > Internet > Opera