O'Donnell was born and raised in Denver and active in student government. He spent his sophmore year at Telluride High where he trained for snowboard racing before finishing high school in Oregon. . He returned to Colorado for college, graduating from the University of Colorado (CU) at Boulder with a degree in International Affairs. While his primary focus in college was on the South American region, it was the geopolitical and worldwide business arenas that caught his attention. Upon graduation he worked as a paralegal and then decided to apply his business acumen to real estate. Following earning his broker's license in 2015, O' Donnell worked for two real estate companies in Denver and then moved to Vail in 2019.

Conversant in Spanish, O'Donnell enjoys snowboarding, hiking, traveling, camping, long distance running and community-based events. He is married to Solange O'Donnell, who he met at CU Boulder.

"Declan's international studies and legal work are excellent platforms for working with our real estate clientele," said Michael Slevin, president of Berkshire Hathaway HomeServices Colorado Properties. "He is a welcomed addition to our Edwards office."