I often wondered that too but upon seeing it tonight again, I wonder if he was talking about his wife - her name was Storm, too, after all. It was a different doc who found O'Malley and told him he had a live cop. Of course, this doesn't rule out the fact that a CPR/Trauma drug kicked in a little late and revived a pulse as suggested in the other replies.
Whatever doesn't kill me only makes me stronger. How strong are you?

Kdarkryder said it exactly. It wasn't a lie, and it wasn't a contradiction. Mason Storm at that point was indeed dead, because his heart wasn't beating. Then O'Malley ran into that second doctor who agreed with the first doctor that Storm had been dead, but then his pulse started again.

He was dead, for a few minutes.
O'Malley asks the same thing, and the second doctor states Mason was dead, but he 'just got a pulse'.