The 74-year-old man was walking along Hurley Close, delivering Christmas cards, between 4pm and 6pm on Sunday when the offence happened.
Police believe that two teenage boys, who passed him on bicycles, flicked at a cigarette at the man.
It caused his oxygen bag to catch fire.
The victim suffered burns to his face, nose, mouth and back.
Police are trying to trace a man and woman, who stopped to help the victim, who could assist with officers' enquiries.
Anyone with information is asked to call police on 101, quoting log number 570 of December 16.