The man, Mark Pittman, had been critically injured when a fire broke out on the rig around midnight. He had been breathing with the help of a tube after surgery was cancelled to help him try to recover.

The fire also killed Gary Keenan, 26, of Ada and Kelsey Bellah, 27 of Tulsah. Pittman died in a hospital in Oklahoma City. Keenan's brother Dewayne, one of the three injured in the explosion, was at home recovering after extensive burns to his hands, with another still hospitalised after the incident.

A GoFundMe page has also been created for the other hospitalised man, Matthew Thurman, which states that 47% of his body has second and third degree burns including his head and face.

"He is in critical condition and facing a long road to recovery," the page says, which has raised over $3000 so far.

On December 23 Pittman's wife, Bobbie Pittman, told media that doctors had done skin grafts on his arms and that his kidneys were not functioning properly.

She told North Texas' KTen news that doctors had told her he was likely to need to stay in hospital for six months if he pulled through, giving him a "40 to 50% chance".

"He's the worst one. He's the most critical. He has 75% of his body burnt and third and fourth-degree burns," Bobbie said, adding that a second worker was also in the same hospital with serious burns, and a third was also hurt.

"DeWayne Keenen, he was injured. He has burns on his hands, from pulling Mark and Matt off the floor," she said.

Her husband had been a motor hand at Dan D Drilling for two years. The drilling rig he had been working on when he was injured is operated under Texas company Pablo Energy. All five employees killed and injured work for Dan D Drilling.

"It's the life of the oil field. You always have those doubts of what's going to happen. It's just like when a police officer goes to work. When they go to work, they have to take their chances," she said, but added that she has tried to keep calm whenever he went into work from 6pm to 6am for his shift.

"They just had a safety test on December 8 and passed it."

While the federal Occupational Safety and Health Administration was still investigating, Bobbie Pittman said there had been speculation on what caused the explosion.

"They had a space heater on the floor," she said.

Dan Darling of the Lamont, Oklahoma-based rig company told KTen news on December 23 that the workers were not supposed to have space heaters, but that he was waiting on the federal investigation.

Bobbie Pittman said her husband did not have any health insurance as the company's insurance option was "too expensive", so friends set up a "GoFundme" account online that has to date raised more than $13,000 for the widow and her family.