You are only required to placard your oxygen vehicle when the weight of both the oxygen and the vessels containing them (cylinders or dewars) is 1001 lbs or over. We advise that you only placard when you are at this weight, as required. If you placard when you are not required to do so, you will have to follow the rules that a placarded vehicle must follow. If you carry 1001 lbs or more, youll need to use the correct placard for the hazardous material (in this case, an oxygen placard). Please note that if you also carry another gas, youll have to use the non-flammable gas placard instead. The oxygen placard may only be used when ONLY oxygen is being transported in the vehicle. You should also place one placard on 4 sides of the vehicle (one on the front, back and sides of the vehicle). For more information on placarding, check out 49 CFR 172.504.