Student B all day long.Undergrad has many many factors such as rigor,major,adjunct classes and a myriad of other extenuating circumstances that impact a GPA. OAT is just a single data point but all others things are equal.Sure,you will have some that had a migraine on test day or others that had the hot hand guessing at answers but a 400 is sharp and gifted.
Plus I would think a good OAT taker will also be a great NBEO taker as well and that is the ultimate end game.