Okolie dominated from the opening round in London, in what was the Brit's first fight since seeing off Dilan Prasovic on the undercard of Anthony Joshua's defeat against Oleksandr Usyk in September.
The WBO cruiserweight champion looked set to end the fight early on after a thunderous few rounds and a knockdown in the fifth.
But his opponent, Poland's Michal Cieslak, survived Okolie's flurry to take the fight to the distance.
Despite a promising start, continous grappling and holding stopped either man from finding rhythym, and Okolie looked far from his best despite the win.
It was something the 29-year-old picked up on following the decision, with Okolie labelling his performance as "messy".
"It was a flat performance and I allowed it to get messy.
"This was one of those days where it was tough, awkward, and the guy was physically strong, but I need to work on the mentality that it doesn't need to be perfect.
"I will go back to the gym, go again and make it right the next time."
Regardless of how he claimed the victory, the scorecard showed Okolie's dominance throughout, with the judges scoring the bout 117-110, 116-111, 115-112.
After a second successful defence of his title, Okolie teased a move up to heavyweight after his win.
When asked about his next possible opponent, he replied, "I will probably have one at heavyweight and then come back down for a unification."