If, like me, you like to install the minimum set of components and then add whatever is required at a later stage, you will soon find that you can not install the Virtual Box Guest Additions : the installation procedure will complain for missing packages.

Actually, the installation procedure is compiling source code, hence it requires some development tools and the kernel header files. None of these are installed by default when you choose a minimal installation.

First Step : add Oracle repositories to YUM configuration (as root)

cd /etc/yum.repos.d
wget http://public-yum.oracle.com/public-yum-el5.repo

Then ensure that the repositories are enabled, i.e. enabled=1 under [ol5_u5_base] and [el5_u5_base]

Second Step : install missing packages

sudo yum install gcc  make  automake  autoconf kernel-headers.i386 kernel-devel

Then you can proceed with normal VirtualBox Guest Additions installation.

Enjoy !