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 -
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