And its not something you necessarily want to do just for the install – because future patchsets and even some one off bugfixes will need to use the old compilers also.

Also note that on Red Hat, you don’t get a choice with oracle word size – if you have built a 64 bit OS kernel, then you have to use 64 bit oracle, and (obviously) vice versa. That’s different to SPARC Solaris, where you can happily run 32 bit oracle on a 64 bit kernel (if for some strange reason you ever wanted to). However it is possible to run a 32 bit kernel OS on 64 bit hardware. “uname -a” tells you whether the OS kernel is 32 bit or 64 bit – if it reports “i386″, its 32 bit; “i386-64″ means its 64 bit.

On RH4, for both 64-bit and 32-bit:
1) Revert back to an old c compiler (as the root userid):

mv /usr/bin/gcc /usr/bin/gcc.orig
mv /usr/bin/g++ /usr/bin/g++.orig
ln -s /usr/bin/x86_64-redhat-linux-gcc32 /usr/bin/gcc
ln -s /usr/bin/x86_64-redhat-linux-g++32 /usr/bin/g++

2) Set environment variable (as the oracle userid before running installer):

export LD_ASSUME_KERNEL=2.4.19

On RH3 64-bit:
1) apply patch 3423540 before running the installer
2) Set environment variable (as the oracle userid before running installer):

export LD_ASSUME_KERNEL=2.4.19

On RH3 32-bit:
1) Revert back to an old c compiler (as the root userid):

mv /usr/bin/gcc /usr/bin/gcc323
mv /usr/bin/g++ /usr/bin/g++323
ln -sf /usr/bin/gcc296 /usr/bin/gcc
ln -sf /usr/bin/g++296 /usr/bin/g++

2) apply patch 3006854 before running the installer
3) Set environment variable (as the oracle userid before running installer):

export LD_ASSUME_KERNEL=2.4.19

(Caveat emptor: I’ve only done installs of RH4 64-bit and RH3 32-bit, so the information for the others is from metalink only.)

References:
RH3 32-bit
RH3 64-bit
RH4 32-bit
RH4 64-bit