The Donegal star announced last month Majella had been diagnosed with breast cancer and asked his fans to pray for her recovery.
The singer has said his wife is a strong and healthy woman who wanted to take control of her hair falling out.
"At the minute she is cutting her hair," he said.
"She is going to have it shaved. She doesn't want her hair to fall out.
"She decided she is going to take charge of her hair going.
"It's very short now and her next step after she has her first treatment she is going to get it all off."
It is understood Majella is recovering after undergoing a successful lumpectomy and was told last week that she will start extra treatment on 12 September.
O'Donnell said reports that his wife was clear of cancer were incorrect, adding that he meant the surgery had removed the cancer but there was still a possibility she would need more treatment, like the majority of patients.
It is not yet known if the 53-year-old will also have to undergo radiation therapy, or if she will decide to undergo surgery to prevent the cancer reoccurring in the future.
O'Donnell, from Kincasslagh, also revealed his mother-in-law Marion will care of Majella when he leaves Ireland this week to tour the United States and Canada.
He made the announcement regarding his wife's illness last month and said they have since been "inundated with tremendous support" from well-wishers.
The 51-year-old said they went public with the diagnosis because she caught the disease early, her outlook was positive and treatable, and they wanted to encourage other people to seek help.
O'Donnell has since urged men and women to get checked out if worried about cancer.
"It's better to go on a fool's errand than wait too long," he said.
The couple met while O'Donnell was on holiday in Tenerife in 1999.
He proposed on Christmas Day 2001 ahead of their wedding the following year.