Judge Neels Claassen said Okah, who was accused of leading the militant MEND group in Nigeria's oil-producing Niger Delta, was found guilty on 13 counts ranging from conspiracy to commit terrorism to detonating explosives.
"The evidence that was given by his accomplices was not contradicted," Claassen told the court in Johannesburg.
MEND, or the Movement for the Emancipation of the Niger Delta, carried out numerous attacks on oilfields and pipelines across the swampy region, which is home to Africa's biggest oil and gas industry, until a government amnesty in 2009. More after the cut
At its peak, the insecurity was costing the OPEC member nation and Africa's biggest oil producer $1 billion a month in lost revenues, according to the central bank.
Security experts believe Okah - who accepted the 2009 amnesty after gun-running and treason charges against him were dropped - was at one time the brains behind MEND although he has denied ever being its leader.
Claassen said Okah, who moved to South Africa after the amnesty and was arrested there, would be sentenced on Jan 30.
The 2010 blasts hit the official celebrations laid on in Abuja for Nigeria's 50th anniversary of independence.
A MEND statement signed Jomo Gbomo, the pseudonym used by the group to claim previous attacks, was sent to news agencies shortly before the explosions, telling people to evacuate the area.