The Oracle Instant Client is a package that brings everything you need to connect to a Oracle Database. You can download it from Oracle with this Link.
With current Intel XEON Servers you should take the x86-64 package. But for 32-bit application you need the x86 package. You can install both in parallel, so it doesn't hurt if you take both. There are also several sub packages. You will only need the "Basic" or "Basic Lite" package if you want to run applications. If you want to build a application (like Perl DBD::Oracle or the PHP Oracle connector) You will also need the "SDK" package. If you want to run sqlplus, you will also need that package. You may also want to pick the right version for your database, but I experienced no problems with the newest client.
If you downloaded all you need the installation is as follows (tested on CentOS 5.2).
# install the RPMs rpm -ivh oracle-instantclient11.1-basic-188.8.131.52.0-1.i386.rpm rpm -ivh oracle-instantclient11.1-basic-184.108.40.206.0-1.x86_64.rpm rpm -ivh oracle-instantclient11.1-sdk-220.127.116.11.0-1.i386.rpm rpm -ivh oracle-instantclient11.1-sdk-18.104.22.168.0-1.x86_64.rpm # set the libraries on the searchpath for the entire system echo "/usr/lib/oracle/11.1/client/lib/" >> /etc/ld.so.conf.d/oracle.conf echo "/usr/lib/oracle/11.1/client64/lib/" >> /etc/ld.so.conf.d/oracle.conf ldconfig # see if the libraries are included ldconfig -p | grep oracle
Now you are ready either to run or build your application.