The Paralympic champion has been found guilty of murder after a South African appeals court overturned an earlier manslaughter verdict.
Oscar Pistorius, 28, killed his girlfriend Reeva Steenkamp in February 2013 after shooting her four times through a locked toilet door.
He is currently under house arrest after spending one year of his original five-year sentence in jail.
Oscar Pistorius will have to return to court to be re-sentenced, for murder.
South Africa’s Supreme Court of Appeal ruled that the lower court did not correctly apply the rule of dolus eventualis – whether Oscar Pistorius knew that a death would be a likely result of his actions.
The minimum sentence for murder is 15 years but judges can apply some discretion.
South African law does not make provision for someone to be placed under house arrest for more than five years, so Pistorius will be going back to prison.
Oscar Pistorius’ family gave a brief response, saying lawyers are studying the finding who will advise them on “options going forward”.
Reading the ruling reached by a panel of five judges, Justice Eric Leach said that having armed himself with a high-caliber weapon, Oscar Pistorius must have foreseen that whoever was behind the door might die.
“As a matter of common sense at the time the fatal shots were fired, the possibility of the death of the person behind the door was clearly an obvious result.
“And in firing not one but four shots, such a result became even more likely,” the judge said.
Oscar Pistorius always maintained that he believed there was an intruder in the house but the judge said that the identity of the person behind the door was “irrelevant to his guilt”.
Justice Eric Leach compared it to someone setting off a bomb in a public place not knowing who the victims might be.
The judge also rejected the argument that Oscar Pistorius had acted in self-defense.
Justice Eric Leach said that Oscar Pistorius’ life was not in danger at the time of the shooting, as the athelte did not know who was behind the door or if they posed a threat.
The judge added that Oscar Pistorius did “not take that most elementary precaution of firing a warning shot”.
The double amputee did not attend the hearing in Bloemfontein.
But Reeva Steenkamp’s mother, June, was present and afterwards she was seen outside the court being embraced by members of the African National Congress Women’s League, who were singing songs of celebration.
Many in South Africa were upset by the original acquittal on murder charges, with women’s rights groups arguing Oscar Pistorius should have been found guilty of murder as a deterrent because of the high number of women who are killed by their partners in the country.
The Paralympic champion was released from prison on October 19. Under South African law, he was eligible for release under “correctional supervision”, having served a sixth of his sentence.
Oscar Pistorius can challenge the ruling in the constitutional court but only if his lawyers can argue that his constitutional rights have been violated.
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