Well You Clicked This Because You Want An oG Tumblr. Follow These Steps! 
STEPS
1: Must Have Tumblr Account
2: Go Here
3: Go To Create URL.
4: Enter OG Name In Subdomain, Click Create Then Copy It.
5: Go To Tumblr Settings
6: Click On Your Blog (On The Left)
7: Check Mark Use Custom URL, Paste The URL.
8: Click Confirm And Wait!
9: Enjoy!
STEPS
1: Must Have Tumblr Account
2: Go Here
3: Go To Create URL.
4: Enter OG Name In Subdomain, Click Create Then Copy It.
5: Go To Tumblr Settings
6: Click On Your Blog (On The Left)
7: Check Mark Use Custom URL, Paste The URL.
8: Click Confirm And Wait!
9: Enjoy!