Defense buff stacking?

[chishi]

I would assume the second option.

[SyndellJigokusakebigoe]

I am fairly certain that it stacks straight across. +20 for Oath, +20 for Rampart, +w/e for Sentinel (I forgot off the top of my head what it is)

During CM runs, I will use both at the same time to tank the Colossi at the very end, and it drastically reduces how much damage I take. Sadly, it is very hard for me to parse average damage I take with or without buffs because I am on PS3, and the Damage Mitigation abilities do not work on the Cactuar Needle attacks.