var q = from t1 in tab1 
                                join t2 in tab2
                                on t1.OtdCode equals t2.OtdCode into joined
                                from t1l.OtdCode in joined.DefaultIfEmpty()
                                where
                                tab1.v = 127
                                group t1
                                by t1.z
                                    into g
                                    select g;

from t1.OtdCode in joined.DefaultIfEmpty()

 var q = from cr in dataContext.CreditRequests 
                                join fil in dataContext.filsb_decodes.DefaultIfEmpty()
                                on cr.OtdCode equals fil.OtdCode            
                                && cr.FilCode equals fil.FilCode