The 27-year-old is due to go on trial on March 3 in Pretoria, South Africa, accused of the premeditated murder of Steenkamp, who died on Valentine's Day last year.
The model was shot dead through a bathroom door at the home of Pistorius, who has maintained that he mistook his girlfriend for an intruder when he fired the shots.
Tatum told The Mirror: "My sister and I had just finished dinner and saw Oscar walk past with two of his friends. I went up to Oscar and nervously asked him for a picture with him.
"He said it would be an honour to take a picture with us and agreed. He was very friendly and polite. He was very calm and was not as self-confident as I've known him to be on TV. One word that describes him aptly that night would be, humble."
The double amputee competed in both the Paralympic and Olympic Games in London 2012.