Okah was convicted of 13 counts related to acts of terrorism.
He was arrested in Johannesburg a day after two car bombs exploded during the 50th anniversary of Nigeria’s independence.
He had denied the charges, even though his militant group, Mend, said it had carried out the attack.
Johannesburg High Court Judge Neels Claassen convicted Okah on charges ranging from conspiracy to commit terrorism to detonating explosives.
“I have come to the conclusion that the state proved beyond reasonable doubt the guilt of the accused,” Judge Claassen said, AFP news agency reports.
Prosecutor Shaun Abrahams told AP news agency that Okah faced life in prison.
South Africa worked closely with foreign law enforcement agencies “to make sure justice is done in Africa,” he said.