Grandmother Lynn Hadley, 67, died in the fire at her end-terrace house in New Street, Shelfield, on April 13 this year.
A jury inquest at Black Country Coroners Court heard today , that paramedics had been called to the house after Mrs Hadley had a high temperature and low mobility.
But as the crew from West Midlands Ambulance started oxygen treatment, sparks from the device set fire to the armchair she was in.
Sadly, despite efforts by paramedics and her husband, Mrs Hadley could not be rescued – and she died in the chair. At a previous inquest, her medical cause of death was given as fatal burn injuries.
A witness statement from Mrs Hadley's husband David, read out in court by coroner for the Black Country Zafar Siddique, said: "At around 4.50pm we called 999. The female paramedic asked her a few questions and took her temperature.
"The paramedics discussed something and then the female took out the oxygen bottle and put it on the armchair. I saw a flash in the corner of my eye. The male paramedic and I tried to pull her [Lynn] as the room filled with black smoke. We were overcome by black smoke – I managed to grab her arm but was unable to do anything."
Mrs Hadley's daughter Kelly, who was also in the house with her daughter, said: "All I saw was sparks – like a Roman candle."
Emma Spencer, a paramedic with West Midlands Ambulance Service, who attended the scene, said: "The symptoms gave Covid concerns. She was quite an unwell lady at that point. Once I found out she had low oxygen levels that is when I asked to my colleague if I should put oxygen on Mrs Hadley.
"After it was turned on, I saw almost immediately there was an ignition – like a box of fireworks or sparklers set off. There were sparks and it just got bigger and bigger – and rather quickly. I thought it was going to blow up."
She told the hearing that it was not uncommon practice to have different sequences of putting the oxygen cylinder together, adding: "I did not realise there was a significant impact on changing the sequence. The manufacturer provides a sequence. The sequence was not a known common thing for administration. I was not aware there was a sequence."
Ambulance technician Steven Kelly, who also attended the scene, added: "I looked over and could hear an almighty sparking noise – in seconds it had taken hold. I could see it was catching fire, my initial thought was to try and move Mrs Hadley in the chair, but we could not move her.
"We both went out via the conservatory. I went back through the property, Mr Hadley came, but we were both pushed back by the flames and the heat, so I had to come back out.
"I was not aware of the manufacturer's instruction. I have never had an incident like this before – I have never heard of an incident like this before.
"It came as a terrible shock to me, this event. I tried to go back into the property, it's somebody's life. It just happened in split seconds – my instinct was to get back in to help this lady."
The inquest, which could last for up to three days, continues.