just flew delta with this situation. they were very helpful….had to fill out a form and when I checked in, my departure city made sure it was good for travel both ways—so my concentrator was cleared. I chose not to use the O2 on return and packed the concentrator in my carry on. When the option to check my bag arose at one point, they were very clear, I could not put my bag with the concentrator in the cargo section…fyi.