As far as I understand, there are no fundamental changes between 6.2 and 6.4 other than errata updates and security fixes. If you require a specific Oracle Linux kernel, you can most probably use the one you need. I think the answer to your question is to analyze why you need to upgrade, and if there is a particular reason, simply test it and the affect to your software. I'm not aware of any documentation that would outline any potential issues for general Oracle database products and upgrading from 6.2 to 6.4. If you want to be sure, there is only one solution. Test it in your particular environment.
To relink binaries is a common practice and should not hurt, but from what I understand it does not really apply to Linux and most modern operating systems that rely on the concept of shared system libraries. Oracle database under Linux from what I know relies on it's own libraries or shared system libraries and therefore I would see no particular need to relink binaries after an operating system upgrade, at least not doing a minor system upgrade from 6.2 to 6.4 and not major from OL 5 to OL 6, which is not officially supported anyway. Part of the idea of shared system libraries is to avoid the need of having to rebuild applications.
1. from db side we can relink the binaries
2. EBS side also we can relink the binaries
I believe you are aware of the steps, however you may see (What Are The Requirements For E-Business Suite When Upgrading Operating Systems? (Doc ID 1061303.1)).
3. OBIEE (don't know)
4. OBIEE uses weblogic (don't know)
Please see (OBIEE 11g: Do Need to Re-link the OBIEE Binaries after Upgrading the Linux Kernel and Kernel Headers. (Doc ID 1554289.1)).