Yes you can travel with oxygen tanks in the car and it's recommended to carry the oxygen tank in the passenger compartment of the car and not the trunk when you can do so.
The reason it's not considered safe to carry an oxygen tank in the trunk of your car is due to a rear end collision which can cause the oxygen tank to explode and injure or kill passengers or the drive in the vehicle.
However sometimes you may have to carry the oxygen tank in the trunk but you should really make sure the oxygen tank is secured and the trunk is ventilated.
Don't leave the oxygen tank in a hot car or a hot trunk of a car because it can easily explode.
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.