The star athlete was sentenced in October 2014 to five years in prison for culpable homicide -- a charge equivalent to manslaughter -- and is now eligible for release into house arrest after serving more than a sixth of his sentence.
Pistorius was initially due to be freed in August.
But intervention by the minister of justice led to a series of delays before the parole board announced Thursday that he would be released on October 20.
If they win their case, which is due to be begin in the Supreme Court on November 3, the 28-year-old could face at least 15 years in jail.
The athlete -- known as "Blade Runner" for the prosthetic legs he wears on the track -- won international fame after racing against able-bodied competitors in the 2012 London Olympics.
His trial was broadcast live around the world.
"I think the chances are pretty good that the appeals court will rule in favour of the state and overturn the verdict," said Ulrich Roux, a criminal lawyer in Johannesburg.
- House arrest -
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Pistorius does not dispute that he shot his model and law graduate girlfriend four times through a locked toilet door in his Pretoria home in the early hours of February 14, 2013.
But he said he mistook Steenkamp for an intruder. Prosecutors insist that he deliberately killed her after an argument.
South African correctional services officials have indicated that Pistorius has been a good inmate and qualifies for house arrest -- a routine procedure in South Africa.
"He's now confined to a house for a period," said Dadic. "They'll confine him essentially to what he would be doing in prison but in the confines of his own house."
Steenkamp's parents June and Barry are outraged by Pistorius's imminent release.
"For our beautiful daughter -- for anyone's life -- it's definitely not long enough," Steenkamp's mother told You Magazine, a South African tabloid.
"She was robbed of her future, her career, her chance to get married and have a baby.