As Kubo announced in Ruby mailing list, with the release of version 1.0.2, the gem of Ruby-OCI8 is available. Now you can easily install ruby-oci8 by simple commd "gem install ruby-oci8".
Though the installation of Ruby-OCI8 has been eased a lot with gem, you still have to pay attention to:
1) The same as installation from source code, Oracle client is still required to be installed before the gem installation. Check help page "Oracle Full Client" and "Oracle Instant Client" on ruby-oci8 website for details, please be specially careful to the setting of environment variable "LD_LIBRARY_PATH".
2) If you don't tell the ruby to load RubyGems every time it starts up by setting the environment variable "RUBYOPT" to "rubygems", you should add "require 'rubygems'" before "require 'oci8'", or you will get error "no such file to load -- oci8".
Now the Ruby-OCI8 gem is only available for version 1.0.2, no support for 2.0 yet.