SQL help doing query inside select

I need to modify some TSQL. Current code does something like this:

select c.rs_id
from a_of a, b_of b, c_of c
where a.o_id=b.o_id and a.bstat IN ("X", "Y")

I need to get a.user_id and use it to find c.mkt_ind. How can I modify this to have a query within a query?

I've heavily modified the original code.  This is just the general idea.