There are many tutorials, but I want to complete with some tricks I just found :

1. in your .bashrc, just add at the end :
   # Oracle instant client
   export LD_LIBRARY_PATH=/home/sinclair/Products/instantclient_10_2
2. in your untargzed directory of ruby-oci8 installation, create a script file with :
   #!/bin/bash
   export LD_LIBRARY_PATH=/home/sinclair/Products/instantclient_10_2
   ruby setup.rb configure
   make
   make install
3. launch this script with sudo command