Hart announced last summer she plans to leave the show-business news program in 2011 after 30 years.
O'Dell, a longtime co-anchor of "Access Hollywood," is to serve as co-host with Mark Steines after Hart wraps up her tenure on "ET."
"I remember watching 'Entertainment Tonight' when it first premiered and dreaming about working for the show someday," People.com quoted O'Dell as saying. "I am honored to be the one chosen to fill a TV legend's seat and I hope I can do her proud!"
"Leaving this job was not an easy decision, but knowing I will pass the baton to Nancy O'Dell makes that decision much easier," Hart said.