Six-time Euroleague winners CSKA Moscow ground out a 74-71 victory at Galatasaray in a fervent atmosphere created by the Turkish fans, with Serbia centre Nenad Krstic scoring a game-high 24 points for the Russian title holders.
On Thursday, Real Madrid enjoyed a rollercoaster 78-74 triumph at Lokomotiv Kuban Krasnodar after overcoming an early 16-3 deficit and Maccabi Electra Tel Aviv scraped a 66-64 home win against European top-flight debutants Bayern Munich.
Milan led for three quarters of the match in the noisy Blaugrana Arena but were undone by a barrage of three-pointers in the final period as Oleson led the fight back and finished as Barcelona's top scorer with 17 points.
Barca lead their Top 16 section with a 6-0 record, ahead of holders Olympiakos Piraeus who have four wins and two defeats after a 78-60 stroll against Anadolu Efes Istanbul.
Krstic hit 11 of his 14 shots from the floor as CSKA held off a strong Galatasaray rally in the closing stages, with British centre Pops Mensah-Bonsu missing a chance to send the game into overtime when he hurled a wild three-point attempt into thin air.
Sergio Rodriguez scored 16 points and fellow playmaker Sergio Llull added 15 as Real took some time to get into their stride in Krasnodar after lifting the Spanish Cup with a dramatic 77-76 win over Barcelona on Sunday.
Llull, who scored a buzzer-beating winner against the Catalan side in that epic contest, was again instrumental as he buried some crucial long-range shots against the unfancied but well-drilled Russian outfit.
"It was a really tough game for us because we made a big effort last weekend during the (Spanish Cup)," Llull said.
"We didn't come out well and it took us a quarter and a half to find our pace, but the important thing is that we beat a really good team on a very difficult court," he said.
Injury-hit Fenerbache Ulker Istanbul kept alive their hopes of reaching the knockout stages with a 98-64 rout of Spaniards Laboral Kutxa Vitoria, whose compatriots Unicaja Malaga celebrated an impressive 87-71 home win against six-time Euroleague winners Panathinaikos.
The Top 16 features two pools of eight teams each, with the top four from each section advancing to the best-of-five quarter-final playoff series.
The playoff winners qualify for the May 15-18 Final Four in Milan.