No it's not oxytocin- there is something specifically for mares that have no bag- I'll go and try to do a search for it. Your Vet needs to wake up, everything may well go fine, and we all hope it does, but s/he needs to be aware that it may not- if the mare has been on fescue her bag will not come in without help, he should know that!!
ETA Domperidone: Oxytocin will not bring the milk in, it only stimulates the womb action and could be dangerous just after foaling as it could cause a prolapse- milk can only be "brought in" if it is actually there- if that makes sense to you! Domperidone will actually start the milk even if there is none present.
Your Vet needs to get this in for you just in case.