The 29-year-old floored the Polish challenger in the fifth round but was not at his fluent best in a bout where both men continued to grapple and hold.
The judges’ scorecards read 117-110, 116-111 and 115-112 – all to Okolie.
“It was a flat performance and I allowed it to get messy,” Okolie told 5 Live Boxing.
“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.”
The Londoner now moves closer to a unification bout, with IBF champion Mairis Briedis a possible opponent.
Raucous crowd but scrappy bout
Prime time boxing on a Sunday evening is a rarity and in the build-up to the main event there was a somewhat more placid, yet intimate, atmosphere compared to regular world-title fight nights.
But the 6,000 fans in attendance certainly made themselves heard when Okolie entered the ring alongside former heavyweight champion and mentor Anthony Joshua and UFC middleweight champion Israel Adesanya.
There was also huge Polish support for Cieslak – the arena at times resembling a football ground with chants exchanged between the home and away fans.
Okolie edged the first round with a huge right which landed cleanly in an otherwise scrappy start.
The fighters engaged in more clinches and smothered each other’s advances in rounds two to four but Okolie – with effective jabs to the body – was doing enough to control the fight and win the rounds.
By the fifth round, Okolie had found his range and looked light on his toes. A chopping right glanced the top of Cieslak’s head and then moments later Okolie countered with another right and the challenger was down on one knee.
Cieslak shook off the knockdown and recovered well in the sixth round. He connected with an uppercut in the seventh and a straight left while Okolie was backing away in the eighth – but the champion was the busier fighter.
Okolie twice landed cleanly in the 10th round but as the fight headed to its conclusion, both men started to get more reckless and the holding seemed to worsen.
Despite the ungainly and often ugly nature of the fight, as the final bell rung there was no question as to who the winner was.
What’s next for Okolie?
It may not have been his greatest performance to date but Okolie is an undefeated world champion with 18 straight wins as a professional.
The 200lb (14.4 stone) division – compared to other weight classes – has historically struggled for depth. With just a handful of elite-level champions and contenders, he may now be provided an opportunity to unify the division and earn his status as the cruiserweight king.
Congolese fighter Ilunga Makabu – who lost to Briton Tony Bellew in 2016 and beat Cieslak in 2020 – holds the WBC belt and could well be the next man Okolie faces.
There is also undefeated Frenchman Arsen Goulamirian – the WBA ‘Super’ champion – but he has not competed in over two years.
Domestically, Chris Billam-Smith is ranked in the top 10 with all governing bodies and undefeated Richard Riakporhe – who has beaten Billiam-Smith – is also in world-title contention.
The most entertaining and profitable fight for Okolie, however, would be a unification battle against the outspoken and often unpredictable Briedis.
“That’s the fight we want to see,” promoter Eddie Hearn said.
The 37-year-old Briedis caused a stir last year when he got a tattoo calling out YouTuber-turned-boxer Jake Paul – an act which alienated boxing enthusiasts and one which Okolie described as “ridiculous”.
Okolie has spoken of the demands of making weight and if the unification fights do not materialise, he could move up a weight class and attempt to emulate fellow Briton David Haye, American great Evander Holyfield and Ukraine’s current unified champion Oleksandr Usyk in winning world titles at both cruiserweight and heavyweight.
Hearn added: “Okolie will move to heavyweight – you will see him at 16 stone plus – but with his ability he can win all the belts in this division and I’d like to see him do that before he makes the move to heavyweight.”
Yafai stars on debut & Gill thrills with ‘Rocky’ performance
On the undercard, Olympic gold medallist Galal Yafai made a huge statement on his professional debut, stopping Carlos Bautista in the fifth round to win the WBC international flyweight title.
It was a classy performance from the 29-year-old Birmingham boxer, who won gold in Tokyo last summer.
Southpaw Yafai showcased quick hands, repeatedly pierced the Mexican’s guard and was also solid in defence against an experienced opponent who himself landed a few clean blows.
Yafai finished it off with a beautiful right-left combination to floor Bautista, with the referee stopping the fight just as the corner were about to throw in the towel.
In an absolutely remarkable fight, Jordan ‘The Thrill’ Gill lived up to his name with a sensational ninth-round knockout win over Karim Guerfi to capture the European featherweight title.
Gill was in control until he was unexpectedly floored by the Frenchman in the seventh round. Guerfi continued the onslaught in the next two rounds, with Gill pinned to the corner and the referee poised to stop the fight.
Then, out of nowhere, Gill unleashed an overhand right and Guerfi hit the canvas. Gill, while celebrating, was still on unsteady legs.
“That was one of the most stunning things I have seen,” promoter Eddie Hearn said.
“It was like something out of a Rocky movie. It was not human what Jordan Gill did there. He never quit.”
Earlier in the night, Anthony Fowler earned a hard-fought 10-round points win over Poland’s Lukasz Maciec and undefeated lightweight Campbell Hatton – son of British and Manchester boxing icon Ricky Hatton – continued his education in professional boxing with a sixth-round stoppage win over Joe Ducker.
There was an explosive debut victory for Olympian Cheavon Clarke, the cruiserweight stopping Croatia’s Toni Visi in the second round with a flurry of punches, while undefeated heavyweight Fabio Wardley knocked out American Daniel Martz in the second round.