More than anything else, Oliver craves social respectability. He has the money, he has the success, but the one thing he doesn't have in life is social standing. That explains why this hard-headed businessman, who's crawled his way out of the gutter single-handed, allows himself to be duped by the Duchess, accepting some pearls from her which he knows to be fake.
As Oliver knows full well, so long as he's involved in a business relationship with the Duchess, even if that relationship is based on bad faith and deception, he'll gain an entrée into respectable society, rubbing shoulders with the kind of people he'd never normally get to know on a personal level. And Oliver believes that through his business dealings with the Duchess, the biggest prize of all will soon be well within his grasp: the Duchess's daughter, Diana, whom Oliver hopes to marry, thus crowning his extraordinary rise to the very top of the social ladder.