O'Donnell announced in her blog earlier this week that she had been treated in hospital for a blocked coronary artery.
The comedian added that she feels "lucky to be here" following the illness.
Trump has now expressed his well wishes to O'Donnell on Twitter, despite their previous public feud.
"@Rosie, get better fast," he wrote. "I'm starting to miss you!"
In return, O'Donnell joked: "well thank u donald - i must admit ur post was a bit of a shock ... r u trying to kill me ? xx."
Trump appeared to signal an end to his feud with O'Donnell earlier this year, describing her as "fun" and saying: "I wish her well."