O'Donnell was originally a swimmer who began competing in triathlon on the advice of his brother while at the United States Naval Academy.

In 2014 O'Donnell announced that he had begun working with Mark Allen in preparation for the 2014 Ironman World Championship.

He finished third in the 2015 Ironman World Championship, where he completed the bike leg behind winner Jan Frodeno, before eventually being overtaken on the run by Andreas Raelert.

In March 2021, O'Donnell suffered a heart attack while competing in the Miami Challenge triathlon.

[4] A year later in May 2022, he returned to racing at the Ironman 70.3 Chattanooga: North American 70.3 Championship placing 6th overall.