How to identify OS is Oracle Linux or RHEL?

There are several ways to identify that, I will suggest one of them using rpm -qf, that finds out what package a file belongs to:

Oracle Linux:

#  rpm -qf /etc/redhat-release
oraclelinux-release-7.8-1.0.7.el7.x86_64

RHEL:

# rpm -qf /etc/redhat-release
redhat-release-server-7.8-2.el7.x86_64