Give you a link

Here is our university's link about O1 status.

http://www.hio.harvard.edu/students_scholars/visas_immigration/o_visa_status.php

O1的审理只需几天，拿到approval（I797)后需要到境外签证回来才能生效（因为你没有waiver,不能再境内转换身份）。

Wish You Luck！