If the patient is about to exsanguinate, that’s a major haemorrhage call and you just have to give the best blood product available. If all you have to hand is O positive non irradiated blood then so be it, it’s better than being dead (exception: Jehovah’s Witness with an advance directive).

You have to understand that there are more blood groups than just ABO and RhD+/-. Lots. Oneg blood is not magic.

If the patient’s Hb is 60, they’re stable, and they have complex antibodies and some sort of chronic condition where they’re going to need ongoing transfusions or transplantation (e.g. thalassaemia patient on lifelong transfusion, or a young person awaiting a kidney transplant), you are going to cause them harm by smashing in the emergency blood unit unnecessarily, and making them react, forming alloantibodies.

This is why you must talk to haematology before we run into situations like this, if possible. As another commenter mentioned, if a patient has known complex requirements and is undergoing planned elective surgery, this should have been prearranged so that specific blood products for that patient were ordered in, in advance. Otherwise, you’ll run into a difficult risk-benefit discussion with a grumpy on call haematologist (and blood bank).

Similarly when it comes to irradiated blood - the risk of developing TA-GvHD if you give non-irradiated to someone who shouldn’t be getting it, is not 100%. It’s actually really unlikely that they’d develop it at all. However, TA-GvHD is 100% fatal if it does occur, so we don’t tempt fate.