he is the only one of the persons in my office---

先行词有the only,the very,the same,the last,just修饰时,只用that.没错/
在the (only) one of +复数名词+定语从句的结构中,only可以省略./
题中是the (only) one of在修饰先行词,而不是the only.所以是可以用WHO的.
he is the only person in my office that knows foreign languages.
he is the only one of the persons in my office that/who knows foreign languages.