It seems that brand name Pitocin requires refrigeration while the generic does not. That's been my experience when ordering - have to get overnight refrigerated shipping from the places that sell the brand names, there is no warning or overnight shipping required from the new place that sells only generic.
I carry everything in my car at all times, with the exception of pitocin. At 36 weeks I leave a little kit of 2 pitocin, 1 methergine, 3 syringes with needles, and alcohol wipes. I put it all in a neat little pencil box that's just big enough to hold it all. I have them put it in the refrigerator door so I know where it is; when I arrive at the house for the birth I take it out and put it with the rest of my emergency supplies. This time of year I don't think it would be necessary, but the drugs in my kit are still within date so might as well stick to the routine.
This time of year, when I arrive I put my IV fluid in a bowl or sink of warm water. I guess it could freeze in my car but it's never happened. I try to keep enough space to keep my car parked in the garage when I'm home.
If the herbs are tinctured in alcohol the temperature shouldn't matter. If they are loose dried herbs the cold will not hurt them (I sometimes store them in the freezer). Heat, on the other hand, would destroy the phytochemicals and make them less potent.
Antibiotic eye ointment should be fine regardless. I'm not sure what to do with injectible vitamin K. It says store at room temp. Oral solutions are supposed to be refrigerated after opening, but again, I carry it with me in my car all the time. I think most midwives make the client buy their own vitamin K so it's not an issue. That works, but I just hate to have the clients pay for a bottle that they are only going to ever use a few drops of.
The oxygen tank itself should be fine, but cold oxygen would make an uncomfortable resuscitation for the baby (hospital oxygen systems often provide heated oxygen). I carry everything in my car regardless. I would rather resuscitate a baby with cold oxygen than not have any because I left the tank at home.