Steps for upgrading Oracle Linux 6.4 to 6.5 and UEK2 kernel to UEK3:

// Download and replace yum repository file with the new one!
# cd /etc/yum.repos.d
# rm public-yum-ol6.repo 
# export http_proxy=proxy.acme.com:80   (optional)
# wget http://public-yum.oracle.com/public-yum-ol6.repo
# nano public-yum-ol6.repo
// find section [ol6_UEKR3_latest] and set Enabled=1
# yum update
# reboot

That’s it. Don’t forget to set Enabled=1 in ol6_UEKR3 section, unless you don’t want to upgrade UEK2 kernel to version UEK3.