Source
The source code for the latest OggConvert release can be downloaded here.
By the magic of Python, you don’t have to install it to run it: just unpack it somewhere and run the oggconvert script.
If you do want to install it though, you can do so by going to the directory where you unpacked OggConvert and running
sudo python setup.py install
Which should give you a nice icon in your Sound & Video menu. You should be aware though that there’s no uninstall facility — you’ll have to go and find the files and delete them youself if you want to get rid of OggConvert again. As such, using a binary package is strongly recommended if one is available for your distribution.