Going back to Mac OS X after installing Ubuntu
The original Apple "OS X Lion USB Thumb Drive" is a full upgrade, so you don't need your old DVD installer disks.

Depending on your Mac and your update history you may even start to Internet recovery mode (hold altcmdR while booting until you see a spinning globe in the center of the screen) and install Lion then. You had to install a special Firmware update before installing Ubuntu to do that though.