Oxygen tanks should not be left in the trunk of a hot car or inside of a hot car.
You can carry oxygen tanks inside the trunk of your car or in the inside of your car as long as you have the windows down or the air conditioning going to keep the oxygen tank cool.
During hot weather make sure you remove the oxygen tank from the trunk of the car or from the inside of the car wherever you have it.
If you leave the oxygen tank stored in a really hot car the oxygen tank could explode and shoot off like a rocket which can kill or injure someone.
Once the oxygen tank reaches a temperature of 125 F or above it can begin to explode and cars can get really hot really quickly.