RubyOSA requires Ruby 1.8.4 or later and Mac OS X 10.4 (Tiger) or later. Ruby 1.8.5 is recommended.
RubyOSA is still under development but it's already complete enough to be useful. The latest release available is 0.4.0.
The easiest way to install it is via RubyGems:
$ sudo gem install rubyosa
There is also a fink package available.
Testing the Installation
Once you have RubyOSA installed on your machine, you can start controlling applications using the instructions in the Guide.