Count Distinct Values

I have two datasets.

Dataset1:

CREATE TABLE #a (Ca nvarchar(20),PERSON NVARCHAR(20));

INSERT INTO #A VALUES ('A','R'),('A','T'),('A','W');
INSERT INTO #A VALUES ('B','R'),('B','M'),('B','R');
INSERT INTO #A VALUES ('C','M')

SELECT * FROM #A

Dataset2:

CREATE TABLE #B (TT nvarchar(20),Ca nvarchar(20));

insert into #b select 'O','A'
insert into #b select 'O','B'
insert into #b select 'O','C'

select * from #b

I insert the table using dataset2 as Dataset Name. However as I can get distinct person in group A,B,C, I do not know how to get total group O person number. Total number should be 4. It would be appreciate that you can provide some ideas.