Yup, on large or sensitive fish, my LFS will use O2 to prepare the bags. The simple fact is that seawater has a very poor capacity for carrying Oxygen. Furthermore, there is actually not all that much O2 in regular air. Being that as the case, large, active, and sensitive fish often times are able to use up all the O2 in the bag, especially when bagged and freaked out. The fish's metabolism goes through the roof and it uses the little O2 in the water and then the little O2 in the air and ends up suffocating or suffering secondary injuries to hypoxia that it never recovers from.
Obviously, O2 bagging is really necessary on the long trip TO the LFS, but is also important on the shorter trip to your house for some fish. And since the fish you're loosing are large, active, and sensitive all in one, not bagging with O2 is a recipie for failure.