O’Brien first joined Ladbrokes in 2004, serving as director of corporate affairs for his entire time at the bookmaker.
Prior to his 10-year spell at Ladbrokes, O’Brien served as a board director at the Red Consultancy agency from 1994 until 2004.
"I’ve had a fantastic 10 years at Ladbrokes but William Hill have offered me a great opportunity to lead their communications as the business continues to expand," O’Brien told the Racing Post newspaper.
"I wish all at Ladbrokes the best and particularly Jim Mullen as he takes the reins."
David Steele, director of corporate affairs at William Hill, added: "I’m delighted Ciaran will shortly be joining William Hill in the newly created role of communications director.
"Ciaran has a wealth of experience in the betting industry, communications, media and public affairs and is a welcome asset to what is already a strong team."