Joshua Steven Durcho, 29, of El Reno could have received the death sentence if he had been convicted at trial, The Oklahoman reported.
Jury selection had been scheduled to begin Monday.
Durcho met privately with the family of Summer Rust before the hearing, a meeting his lawyer called "very emotional." He wept in court as relatives read statements describing how the killing of Rust and her four children had affected them.
Rust, 25, and her daughters -- Kirsten Rust, 7; Autumn Rust, 7; and Evynn Garas, 3; and son, Teagin Rust, 4 -- were strangled in her El Reno apartment in 2009. Investigators said Durcho killed her because she threatened to call police, and killed the children to eliminate them as witnesses.
Canadian County Sheriff Randall Edwards said he did not believe there should be a plea in the case.
"I don't believe a crime so heinous as the one Durcho is accused of committing should be [pleaded] out, certainly the five victims he is accused of having strangled to death, four of which were babies, did not get to make a plea for their lives," Edwards said before the hearing. "There is overwhelming evidence that shows he committed this horrific act and then ran like the coward he is. I believe he had ought to stand before a jury of his peers so at least if he is found guilty, the death penalty will be, should be, an option."
Durcho will not be eligible for parole under the terms of his plea.