In a statement posted in his website, the South African athlete said, "No words can adequately capture my feelings about the devastating accident that has caused such heartache for everyone who truly loved — and continues to love Reeva."
"The pain and sadness — especially for Reeva's parents, family and friends consumes me with sorrow.
"The loss of Reeva and the complete trauma of that day, I will carry with me for the rest of my life," he said in the message he simply signed "Oscar".
He also posted a link to the statement on Twitter, describing it as "A few words from my heart". It was the first time he had used the social networking site since 12 February, 2013, two days before the shooting.
Oscar is due to go on trial for Reeva's murder in Pretoria in March. He claims that he mistook her for an intruder when he shot her through the toilet door on Valentine's Day 2013.
Oscar with his late girlfriend Reeva
The prosecution alleges that the killing was premeditated following an argument between the couple. If the Paralympian is found guilty, he could face life imprisonment.
Reeva Steenkamp's uncle has said there will be a private family gathering near Cape Town to mark the anniversary of her death, though her parents and brother would not attend.
"We will release balloons in her memory," Michael Steenkamp told AFP. "They will be red and white, her favourite colours."