People with O- (read "O negative") blood are referred to as universal donors. The red blood cells in O blood have no antigens, unlike all other blood types. The lack of antigens means people with other blood types won't react to O- blood, so O- blood is almost as good as receiving your own blood type. Only 7% of the population has 0- blood, which makes the universal donor somewhat rare.
If your surgeon felt that you needed blood before surgery, and this wasn't part of the plan for your procedure, O- blood would be used so that the blood could be given more quickly.
To give you blood that exactly matches your type requires drawing your blood, sending it to the lab and having blood prepared for you. This process, commonly referred to as a "type and cross," takes time. In emergency situations, or when time is of the essence, O- blood can be given safely while the process for matching blood is started (should you need more later on).
In addition to blood transfusions, organs from a person with type O blood (either O+ or O-) can be transplanted in people with any blood type, although additional tests are necessary to ensure a safe match. For plasma, AB- is the universal donor type.
50 Quick Facts. The American Red Cross. Accessed October, 2010. http://www.givelife2.org/sponsor/quickfacts.asp