I am taking a left outer join to which I would like to add a
condition.

create table tmp_nani_student(stid number) ;

insert into tmp_nani_student values(1) ;
insert into tmp_nani_student values(2) ;
insert into tmp_nani_student values(3) ;
insert into tmp_nani_student values(4) ;


create table tmp_nani_marks(stid number, marks number, grade number) ;

insert into tmp_nani_marks values(1, 51, 3) ;
insert into tmp_nani_marks values(2, 71, 2) ;
insert into tmp_nani_marks values(4, 81, 1) ;
insert into tmp_nani_marks values(5, 55, 3) ;
commit ;

Then I run :
select a.stid, b.marks, b.grade
from tmp_nani_student a, tmp_nani_marks b
where a.stid = b.stid (+) and (b.grade < 3 or not exists (select
c.stid from tmp_nani_marks c where a.stid = c.stid) )

So far so good.

But then I add one more row to the second table:
insert into tmp_nani_marks values(2, 17, null) ;

Now I want to run a query which would exclude this new row where
grade=null. If the outer join fetches rows with grade=null, that is
fine. But I don't want to fetch records where grade is explicitly set
to null.

Run again:
select a.stid, b.marks, b.grade
from tmp_nani_student a, tmp_nani_marks b
where a.stid = b.stid (+) and (b.grade < 3 or not exists (select
c.stid from tmp_nani_marks c where a.stid = c.stid) )

But the record (2,17, null) shows up. I don't want it to show up. What
would be the query for it?

TIA.