When my wife needed oxygen on board, we were able to hire a concentrator for use in the cabin. The company we used were well used to working with Cunard and both delivered the machine to the cabin and and collected it afterwards. For us it worked very well. It was discreet, quiet and removed any worries about continuity of supply and transport issues. However, pre-voyage, we did have to complete a Mobility and Medical Questionnaire for Cunard's Accessibility Services department. They gave us useful advice and can be reached at email@example.com.