The Balsa prototype based on Ubuntu 9.04 (jaunty) is available for alpha testing. This is the only version that works for the OLPC XO-1. It is in the form of an image file that you can download and copy to an SD card.

You can check what you will need for this version.

You have to use a non-XO machine to download and build the SD card.

If you use a Windows machine, a link for downloading the SD card image and instructions for making the card are here.

If you use a Linux machine, the link is here.