Because I work for a volunteer service that consists of both fire and EMS in a fairly rural area, along with the private service I work for, I do carry a fairly large first in/o2 bag which contains:
O2 cylinder/regulator
02 masks and nasal canulas (adult and Pedi)
2 full OPA sets and 1 Full NPA set with lube
Adult BVM
Sam Splint
Trauma supplies (2x2s, 4x4s, 5x9s, kling, band-aids, multi trauma dressings, hot and cold packs, etc)
Protocol book
Space blanket
Tissues
CPR masks (2)
Box of gloves
I carry this in addition to my FD bunker gear and EMS turnouts with consist of: turnout pants, turnout coat, work/extrication gloves, and 2 helmets ( one for the volunteer EMS and the other for the private EMS service. I also carry my pager and portable radio