The 35-year-old collapsed during his team's game on Saturday and was pronounced dead a short time later.
Robert Liddle, the club doctor, said: "The post-mortem revealed that Phil had suffered left ventricular failure of the heart."
It was also announced that the player's funeral service would take place in Hamilton at noon on Friday.
The Requiem Mass, in St Mary's Church, Cadzow Street, will be followed by interment at the nearby Bent Cemetery.
At the request of his wife Eileen, the church service will be for family and friends only.
Mrs O'Donnell said she had been left "devastated" by her husband's death.
"He would like to be remembered as a family man"
Speaking publicly for the first time since he died, she said he had achieved much in football but "the most important thing for him was his family".
"He would like to be remembered as a family man and we were all so proud of him," she went on.
"We are all devastated by what has happened.
"We hope we will now be given peace and time to grieve but we are overwhelmed by the messages of support we have received."
The player collapsed and died as he was substituted during his side's 5-3 defeat of Dundee United at Fir Park on Saturday.
Celtic manager Gordon Strachan and Rangers captain Barry Ferguson were among the figures from across the world of football who visited Fir Park to see the tributes.
Wednesday's Old Firm derby between Rangers and Celtic, O'Donnell's former club, was postponed as a mark of respect for the father-of-four.
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