Okah, 46, was convicted of 13 terrorism charges, many of them stemming from the 2010 twin bombings in Abuja that killed 12 people, Nigeria's online newspaper the Daily Post reported Wednesday.
He is thought to be the first foreign national to be tried for terrorism in South Africa.
State Prosecutor Shaun Abrahams said Okah was also given a 10-year prison term for being a threat to South Africa that will run concurrently with his 24-year sentence.
Okah's conviction and jail sentence have been condemned by his armed militant group Movement for the Emancipation of the Niger Delta.
MEND has threatened "hell" over Okah's conviction, the Premium Times of Nigeria reported.
The group described his sentence as "the height of injustice to our region and people."