When Yosef was appointed Second in Command in Mitzrayim he suddenly became the most eligible bachelor in the land. How did the shidduch of Yosef and Osnas come to be? Why did he agree to marry Potiphar’s daughter after he had set him to prison for years?
The Daas Zekeinim give two explanations. The first answer is that it was pure blackmail. They needed a way to silence Potiphar about Yosef’s past as a slave. Once his daughter married Yosef he would have every incentive to keep quiet.
The second answer follows the pshat that Osnas was Dina’s daughter (see another dvar Torah “who was Osnas.. Really??” also on this site). According to that pshat she had an amulet around her neck that said she came from the house of Yaakov. Yosef was so good looking that wherever he went women would throw their jewelry towards him. Osnas threw her amulet at him and when he read it he sought her out for marriage.