He is accused of killing her in the early hours of Valentine's Day 2013 following an argument. Oscar has said he shot Reeva after mistaking her for a burglar.
Addressing her mother June directly in a voice shaking with emotion, the Paralympian said that there "hasn't been a moment since this tragedy happened that I haven't thought about your family".
"I wake up every morning and you're the first people I think of, the first people I pray for. I can't imagine the pain and the sorrow and the emptiness that I've caused you and your family.
"I was simply trying to protect Reeva. I can promise that when she went to bed that night she felt loved. I've tried to put my words on paper many many times to write to you. But no words will ever suffice."
He said he had lost a lot of weight since last year and is taking anti-depressants and sleeping pills.
"I'm scared to sleep, I have terrible nightmares, I can smell blood and wake up terrified," he said.
He added that he never wants to handle a gun again. At times Oscar was crying so much he was asked to speak up by Judge Mapisa and his own barrister Barry Roux.
Many people in the packed courtroom were unable to hold back tears, while the judge and her two legal assistants stopped taking notes during his statement instead listening intently.
Reeva's mother June betrayed no emotion during his testimony. Mike Steenkamp, Reeva's uncle, had earlier said she wanted to be in court to hear what the Paralympian had to say "for closure".
The sprinter went on to describe a boating accident in 2009 and its impact on his state of mind. He had to have 170 stitches to his face, and the incident profoundly changed his outlook on his life and left him more vigilant about his own safety, he told the court.
"I was a lot more vigilant about losing my life after that... I became quite withdrawn," he said. "People were joking about it. People didn't understand that I'd nearly lost my life."
Speaking about his late mother, Oscar became emotional once more - prompting his brother Carl and sister Aimee to sob.
He described how after the marriage of his parents broke down, his mother was concerned about living on her own.
Oscar said: "My mother had a lot of security concerns. We grew up in a family where my father wasn't around much so my mother had a pistol.
"On a couple of occasions people did break into our home... She kept her firearm under her bed, under her pillow in a padded leather type of bag."