Ali Rowghani, once Twitter’s chief operations officer, is joining startup accelerator Y Combinator as a part-time partner, the accelerator announced today.

Rowghani, who’s also been Pixar’s chief financial officer, will be mostly working with alumni companies that are now dealing with the challenges related to growth. Y Combinator president Sam Altman writes in a blog post:

Though we’ve traditionally focused on helping very early-stage companies, our successful companies have asked for help on topics like scaling operations, managing hypergrowth, building out management teams, etc.

Ali is a perfect fit for helping YC companies with these scaling questions. Ali was the CFO at Pixar, where he spent 9 years working closely with Ed Catmull and Steve Jobs. In 2010, he joined Twitter as its first CFO, at a time when Twitter had about 100 employees, 20 million users, and virtually no revenue. In late 2012, he became Twitter’s COO and took on many additional responsibilities within the company.

Rowghani left Twitter this past June, stepping down as chief operating officer but remaining as a strategic advisor. His resignation was rumored shortly before the announcement, and he was allegedly taking the heat for several issues, including the company’s lack of significant growth and innovation, and his sale of 300,000 shares of Twitter stock. Then newly hired product vice president Daniel Graf was also told to report directly to chief executive Dick Costolo instead of Rowghani, signaling tensions in the management structure at the time.

Rowghani will be joining Y Combinator’s other part-time partners Adora Cheung, Patrick Collison, Elizabeth Iorns, Andrew Mason, Yuri Sagalov, Emmett Shear, Michael Seibel, and Harj Taggar.