Hope someone with current knowledge of 121 regs can help with this one. My mother who is on medical oxygen at her MD's insistence, was refused boarding with her small carry on size bottle. She was subsequently informed that if required, the airline would have to supply it, and she would be charged 100 dollars per flight segment, for it's use. Does anyone know if carrying portable oxygen on board is governed by 121 regs, or is it governed by individual company policy ? I am familiar with 135 regs regarding this, but evidently these rules do not apply to scheduled 121 ops. Thanks for any input.