Apart of modifying
/etc/redhat-release to "redhat-4", there are a couple of packages that you will need to install in order to make Oracle 10g work with Cent OS 5.1:
yum install \
compat-db-4.2.52-5.1 \
glibc-devel-2.5-18.el5_1.1 \
libX11-devel-1.0.3-8.0.1.el5 \
libXau-devel-1.0.1-3.1 \
libXdmcp-devel-1.0.1-2.1 \
libXmu-1.0.2-5 \
libXtst-devel-1.0.1-3.1 \
mesa-libGL-devel-6.5.1-7.5.el5 \
openmotif-2.3.0-0.3.el5 \
xorg-x11-proto-devel-7.1-9.el5.centos