In an interview with the Mirror, June Steenkamp said she has been studying the star athlete obsessively to learn the truth about the night that he says he mistook his girlfriend for an intruder.
She said she was "unmoved" by the Paralympian's apology at the start of the week. "I knew it was coming."
June, who had never met the sports ace before the hearing, added: "He has been very dramatic, the vomiting and crying..."
There is one burning question the griefstricken mother is asking herself: "I don't know whether he's acting."
"I look at Oscar the whole time, to see how he is coping, how he is behaving. I'm obsessed with looking at him, it's just instinctive, I can't explain it," she told the Mirror's Lucy Thornton.
She said doesn't care if Oscar is cleared or sent to prison as it won't bring back her daughter, but says "he has to stand up to what he's done and - if he has to - pay for it".
In court June has mostly maintained her composure by "being strong for Reeva", but back at her hotel it's a different matter, she explained. "It all comes out and I break down."
On Thursday, the sprinter was questioned in the witness box about his relationship with Reeva, who was a law graduate as well as being a cover girl.
Oscar denied "picking on her" or throwing tantrums in public and added that he regretted not telling her that he loved her during their four-month relationship.
There has been a lot of focus during the hearing on the phone messages that passed between the two.
Chief prosecutor Gerrie Nel asked the athlete about one message over an argument following an awards ceremony, trying to paint a picture of him as moody and egocentric. "It was all about you Mr Pistorius," he said.
Oscar replied that Reeva was a person who always stuck up for herself, if she thought he had treated her badly she wouldn't have been with him. "This was an argument that lasted one message," he said.
The prosecutor also questioned why the runner had apologised to the model's parents in public with the whole world watching - rather than in private.
He never got the opportunity, he responded, and couldn't find the right words.