Oxygen levels are irrelevant to the basic BP issue in tall sauropods. Either
the blood gets up there or it doesn't, it's a matter of the mass density of
water-blood.
I was responding to Sim Koning's comment about "a constant oxygen supply"
for the sauropd's brain. If oxygen concentration were higher, the blood
would be more oxgenated and it could better "ride out" a blip in the
supply.