French President Nicolas Sarkozy was among the heads of state and dignitaries who attended the event in the former French colony's largely ceremonial capital Yamoussoukro.
He also addressed some of the thousands of French citizens living in the commercial capital Abidjan.
Ouattara was declared winner of a UN-certified election last November billed as a chance to reunite the fertile, cocoa-growing West African nation, after rebels seized its northern half in late 2002.
Instead, the country lurched back into civil war when incumbent leader Laurent Gbagbo refused to step down and used troops, paramilitaries, violent youth militias and Liberian mercenaries to entrench his position and crush dissent.
At least 3,000 people were killed and more than a million displaced in the power struggle, in which cocoa exports ground to a halt, banks shut and shops were ransacked by militiamen.
The impasse ended when pro-Ouattara rebels backed by the French military raided Gbagbo's compound at the height of the fighting and seized him from his blast-proof bunker.
UN Secretary-general Ban Ki-Moon said the international community should support Ouattara. "A democratically elected person should be the one to lead the country based on the will of the people. This is what we have learned, this is what we have to send out," he said.