Question of Excel formula if nested error

I would like to make a formula on excel file which require to meet the following requirements:

1. If N column is empty, then nothing to do
2. If N column are not C and O, then show message of "invalid value" (Remark: A,B,D,E.....M,N,P,Q....Z)
3. If N column is C and T column is empty, then show message of "please write something here"

After run, it prompt up the error message of "You've entered too many arguments for this function". May I know what's wrong with me?

Thanks.

=IF(ISBLANK(N815),"", IF(OR(N815="C",N815="O"),"","Invalid Value!", IF(AND(N815="C",ISBLANK(T815)),"Please write something here!")))