1.He is one of my friends who are working hard.
引导词who做定语从句的主语,此句中先行词为friends,所以从句中谓语用复数《are》
2.He is the only one of my friends who is working hard.
引导词who做定语从句的主语,此句中先行词为the only one ,所以从句中谓语用单数.
你就记住这个规律:
当one of +名词复数,后跟定语从句时,从句谓语用复数.
当one of +名词复数,前面有the only|the very修饰时,后面的定语从句谓语为单数