1. The opening of a patient's mouth into a circle, like the letter O, for a prolonged period of time, indicating that they are near to death. We rushed through the wards of injured soldiers, prioritizing the ones wearing the O-sign first.

2. A laparoscopic adjustable gastric band that has slipped out of position, which shows on a radiograph as a distinct O shape. We're taught to look for the O-sign whenever we do scans of patients who have been fitted with a lap band.