Rosencrantz takes up her first full time position since leaving Virgin Media TV, where she was director of television, in January 2011 after the company was bought by Sky for £160 million.
According to Broadcast, she will take up a newly-created role at indie producer Fresh One, which started as the vehicle for Oliver's food programmes but has since expanded to other factual shows fronted by the likes of Jonathan Dimbleby and Idris Elba.
Rosencrantz, who spent a decade as ITV's controller of entertainment and helped develop shows such as Who Wants To Be A Millionaire, Pop Idol, The X Factor and Hell's Kitchen, will now focus on bringing new talent to Fresh One.
In a statement, Oliver said of Rosencrantz: "Her track record in making world-class entertainment and drawing in and growing the very best talent speaks for itself."
Fresh One managing director Roy Ackerman added: "She's a producer who loves making programmes and we want Fresh One to have an array of top programme-makers to execute our formats and series.
"So it's an exciting opportunity to move properly into entertainment and a range of shows at the entertainment end of factual entertainment."
Rosencrantz praised celebrity chef Oliver for building a production company and a brand "that is synonymous with accessibility, quality and integrity".
She added: "What brilliant foundations upon which to build the next phase of programming and talent with Roy Ackerman and the fantastic team at Fresh One."