Oulaye, 64, is accused of using intermediaries to provide two million CFA francs ($5,300, 4,500 euros) for purchasing arms for men who carried out the attack.
Seven Nigerians in the UN peacekeeping mission in Ivory Coast, known as ONUCI, and eight civilians were killed in the assault near Guiglo, in the west of the country, in June 2012 — an incident that stoked fears of instability in the fragile country.
Oulaye was former minister for the public sector and the government’s spokesman in the first decade of the 2000s.
He was a key figure in Gbagbo’s regime, which plunged Ivory Coast into bloody unrest.
About 3,000 people died when rival supporters clashed after Gbagbo refused to accept defeat by bitter rival Alassane Ouattara in presidential polls in November 2010.
Gbagbo was eventually ousted the following April, when he was arrested with the help of UN and French troops, and handed over to the International Criminal Court, where he faces four charges, including murder, rape and persecution.
Oulaye served more than two years in preventive detention from May 2015 to June 2017.
His lawyer says the charges are baseless and his client is the victim of a “political trial” aimed at stirring tensions within Gbagbo’s old party.
His Ivorian Popular Front (FPI) is divided into two factions, between pro-Gbagbo hardliners and those who are inclined to turn the page on the Gbagbo past.