In an absolutely dire, no coss-matched-blood-in-the-fridge-and-the-patient-is-about-to- sprout-wings-and-a-halo emergency you would give O Neg. Every hospital blood fridge has 2 units of O Neg at any one time for such an eventuality.
If you are pushed yes.But due to the risk of producing anti-D antibodies in rhesus neg patients its probably best to avoid it if you can.All hospitals have O neg blood available for emergencies, but if there is major incicdent,eg bomb/train crash, give O pos blood preferably to old people(less likely to have active immune systems) and men.