Update: FirstClass is apparently not used anymore with the OU and has been replaced with a Moodle based system. It has only a web interface (no client software like First Class), so all you need is a web browser.
FirstClass is a conferencing and collaboration system used as a prior means of communication with the OU. The Online Applications CD only ships a Windows version, but it is cross plattform.
Linux and Mac[]
Get it on the FirstClass download page.
After installing it, you will have to configure the OU servers (this is not necessary for the Windows version installed from the CD).
I've uploaded the necessary configuration files found on the CD (if you want to copy them from the CD yourself, please note that one had a case sensitivity problem).
There are two OU FirstClass servers, the second one is for technical and science students, the first for all the other guys (you are welcome to edit and elaborate on this).
Extract the downloaded file. Fire up the program. Where you see "home.cf", click the browse button near. Select the fc file you just extracted and login.
Now have fun with FirstClass!