The Oracle 64-bit client has not worked since OSX Lion was released in 2011. This is a problem as Ruby on Lion is 64-bit. Rather than recompiling Ruby to be 32-bit its much easier to run JRuby and use a jdbc connection string.
A sample database.yml
:adapter => 'jdbc', :driver => 'oracle.jdbc.driver.OracleDriver', :url => 'jdbc:oracle:thin:@your.oracle.server.com:1521/something.world', :username => 'user', :password => 'password'