“I have come to the conclusion that the state proved beyond reasonable doubt the guilt of the accused,” said Judge Nels Claassen, handing down the verdict in the South Gauteng High Court.
Okah was found guilty of masterminding attacks including twin car bombings that killed 12 people in Abuja on October 1, 2010 and two explosions in March 2010 in the southern Nigerian city of Warri, a major hub of the oil-rich Delta region.
The Movement for the Emancipation of the Niger Delta (MEND), a group fighting for a greater share of the Delta oil wealth, claimed responsibility for the attacks.
Okah denied involvement in the blasts and said the charges were politically motivated.
He also denied leading MEND, but had said he sympathised with their goals.
However the South African court found Okah was the leader of the movement after uncovering documentary evidence including his wife’s handwritten notes.