Finding Linux OS Version
 

When I use cat # /etc/version I get the following line:
Linux version 2.6.18-194.el5xen (mockbuild@ca-build10.us.oracle.com)(gcc version 4.1.2 20080704 (Red Hat 4.1.2-48) )

But when I run # cat /etc/redhat-release I get
Red Hat Enterprise Linux Server Release 5.5 (Tikanga)

and when I run # cat /etc/issue I Get
Red Hat Enterprise Linux Server Release 5.5 (Carthage)

My problem is: what is the red hat version I am using ? bcoz, I want to install Oracle DB 12.2, so what installation guide should I refer, Linux 4 or Linux 5 ? Please help me ? Thanks.

5.5. That's the only part of any of that you need to care about 95% of the time.

That's definitely RHEL 5. Tikanga is the RHEL codename. I believe Carthage is the Oracle codename for their version (ie rebuild) of RHEL5.
Is this a pure RHEL build or have you added to or partially installed from Oracle aka OEL?

This is a pure RHEL.

One more confusion.

if Red Hat Enterprise Linux Server Release 5.5 (Tikanga) is showing the version of Linux OS, then

what is gcc version 4.1.2 20080704 ?

What is Red Hat 4.1.2-48 ?

The GCC version the kernel was built with.

To expand on the above

gcc version 4.1.2 20080704

is the gcc version, inc date of release.

Red Hat 4.1.2-48

is the RH (patched) version of the above kernel

uname -a and cat /etc/issue are all you'll need to read to identify your distro's version.

That is true for several distros but not for all. For a mature and generic technique, see rkhunter's rkh_dat_get_os_info function.