At metalink (oracle support) I found a document "Linux OS Installation with Reduced Set of Packages for Running Oracle Database Server" [ID 728346.1]. In this document it stated that no additional tasks should be included when installing Enterprise Linux. So, this time I also didn't check 'Software Development' (besides Web Server, Clustering and Storage Clustering).

Now, At a next step, when trying to install the VirtualBox Guest Additions I got the message "Building the VirtualBox Guest Additions kernel modules [FAILED]". The content of /var/log/vboxadd-install.log:
Makefile:23: *** Error: unable to find the sources of your current Linux kernel.
Specify KERN_DIR= and run Make again..

As a result, I was not able to mount a shared drive. So I had to solve this first. I installed all packages from the Oracle Linux 5 distribution media mentioned in the document "Oracle E-Business Suite Installation and Upgrade Notes Release 12 (12.1.1) for Linux x86-64" [ID 761566.1]:

cd /media/Enterprise Linux dvd 20100405/Server
rpm -i --nodeps --replacefiles xxx where xxx stands for

gcc-4.1.2-48.el5.x86_64.rpm
gcc-c++-4.1.2-48.el5.x86_64.rpm
glibc-2.5-49.i686.rpm 1)
glibc-2.5-49.x86_64.rpm 1)
glibc-common-2.5-49.x86_64.rpm 1)
glibc-devel-2.5-49.i386.rpm
glibc-devel-2.5-49.x86_64.rpm
libgcc-4.1.2-48.el5.i386.rpm 1)
libgcc-4.1.2-48.el5.x86_64.rpm 1)
libstdc++-devel-4.1.2-48.el5.i386.rpm
libstdc++-devel-4.1.2-48.el5.x86_64.rpm
libstdc++-4.1.2-48.el5.i386.rpm 1)
libstdc++-4.1.2-48.el5.x86_64.rpm 1)
make-3.81-3.el5.x86_64.rpm 1)
gdbm-1.8.0-26.2.1.i386.rpm
gdbm-1.8.0-26.2.1.x86_64.rpm 1)
libXp-1.0.0-8.1.el5.i386.rpm
libXp-1.0.0-8.1.el5.x86_64.rpm
libaio-0.3.106-5.i386.rpm 1)
libaio-0.3.106-5.x86_64.rpm 1)
libgomp-4.4.0-6.el5.x86_64.rpm
sysstat-7.0.2-3.el5.x86_64.rpm
util-linux-2.13-0.52.el5_4.1.x86_64.rpm 1)
compat-libstdc++-296-2.96-138.i386.rpm 1)
compat-libstdc++-33-3.2.3-61.i386.rpm 1)
compat-libstdc++-33-3.2.3-61.x86_64.rpm 1)
elfutils-libelf-devel-0.137-3.el5.x86_64.rpm
elfutils-libelf-devel-static-0.137-3.el5.x86_64.rpm
libaio-devel-0.3.106-5.x86_64.rpm
unixODBC-2.2.11-7.1.i386.rpm
unixODBC-devel-2.2.11-7.1.i386.rpm
unixODBC-2.2.11-7.1.x86_64.rpm
unixODBC-devel-2.2.11-7.1.x86_64.rpm
kernel-headers-2.6.18-194.el5.x86_64.rpm

1): were allready installed (message).

After installing all those packages I still got the same error message. Surfing on the internet I found out that I had to install the kernel-devel-2.6.18-194.el5.x86_64.rpm (not mentioned in [ID 761566.1]). After installation of that package, I managed to install the VirtualBox Guest Additions.

Also see my page "Installation of Oracle Enterprise Linux 5.5 in VirtualBox". I added it some weeks ago, but as I am still preparing the Oracle Linux environment for installation of EBS R12.2, the document is still under construction.