Tachibana is in a coma, but when she wakes, who will she be?
Otonashi stays by her all night. When he falls asleep, he dreams of his own death.
He didn't die in the crash, or at least, not immediately.
It was a cave-in, and the passengers were trapped underground for 7 days.
He was my injured internally in the crash, and though he organized all the survivors and bandaged their wounds, he did not survive until the rescuers arrived.
The last thing he did was fill out his organ donor card. He couldn't become a Doctor, but he at least saved one life.