Can I ask what product requires a specific version? since old products can require one for compatibility, however the manual will point usually to OSS.oracle.com for products like old ias, e-business suite, etc.
but for actual products, they usually list the minimum tested version, and any higher than that will be ok, as oracle support OS patching as a normal best practice.
You can download the RPM packages from the Oracle Public Yum repository, but why would you want do this?
Since you are asking about the RPM package I'm wondering if you are aware that YUM is the standard package management utility, which unlike the old RPM utility automatically resolves software dependencies and prevents you from installing conflicting software. It is standard since RHEL 5.
The distribution package you are looking for should also be on the normal OS installation DVD.
So to install the software you need, you could simply setup access to the public YUM repository following the instructions at http://public-yum.oracle.com/
Or in your case:
# cd /etc/yum.repos.d
# wget http://public-yum.oracle.com/public-yum-el5.repo
# yum list openmotif*
# yum install openmotif22
Alternatively, you can also configure yum to read the standard OS installation DVD.
Using the YUM utility you can easily find out what software is available and query the YUM database and information. When using YUM to install openmotif22 it will automatically download the required libX packages and correctly install the software.