Nichols admitted killing Flippo, telling Special Judge Larry A. Jones: “I deliberately stabbed Jenna Flippo to death. I am so sorry.”
Neither Nichols nor his parents, who stood behind him when he entered his plea, would comment outside of court.
Nichols and Flippo had two children together but were separated, police said.
Court documents show Flippo had been abused in 2005 by Nichols, who served prison time as a result. She also told police he threatened her in November, a couple of months before she was killed.
Flippo's death was Oklahoma City's first homicide of 2013.
Her father, stepmother and brother were present Friday but declined to comment inside or outside of court.
“We just felt like it was in everyone's best interest to bring some finality to this case,” Rowland said.