I'm the only one who can do it right .这句话是定语从句吗?我记得老师说过先行词被the one或the only修饰时,只能用that引导定语从句,

应该是这样：
1. 当先行词是the only (“唯一的”),the very (“恰恰；就”),the just (“恰恰；就”),the same (“相同的”),the last (“最后一个”)等或被这些词修饰时, 只能用that,如 The only thing that I need to do is to cook a meal. / He’s the very person that I want to find. / Did you miss the last problem that is in your math test?
2. 当先行词是one, someone, anyone, everyone, no one或these, those以及he, they时,只能用who.如 Anyone who came late must tell the reason. / Those who came late are from Class One. / He who doesn’t go to the Great Wall is not a true man.
3. the only one 的中心词是one,所以,只能用who.