On Oct. 1, 2010, a car bombing in Abuja left 12 people dead and 36 injured in an attack that coincided with Nigeria's Independence Day.
A South African court ruled Monday that Okah was guilty on charges of terrorism stemming from his role in the attack, the Mail and Guardian (Nigeria) online newspaper reports. He was arrested in Johannesburg the day after the attack.
Okah is considered the leader of the Movement for the Emancipation of the Niger Delta. The rebel movement has attacked oil facilities in the region. MEND says Delta residents aren't getting their fair share of state oil revenue.
Nigeria is among the top oil exporters in the region. Nigerian crude oil production rose to more than 2.1 million barrels per day following a 2009 amnesty for Nigerian rebel groups.
The U.S. Energy Information Agency, however, reported recently that crude oil deliveries from Nigeria were down for the summer about 500,000 barrels compared to the same time last year.
Flooding in the oil-rich Niger Delta prompted Shell and French supermajor Total to hold back on exports from the country last year.