I have never used optima, just make sure the size is correct for the ML's battery box, i doubt it is due to it's odd shape, I have seen ill fitting batteries cause fires when not properly installed, especially on a 4x4's.
the Mercedes batterywill fit's the ML properly most aftermarket batteries are not tall enough and will not allow the little plastic wedge on the top to lock in place then the battery will move because it is not properly secured.