An inline view will do the trick:
SQL> create table t ( a int, b int, c int, d int );
Table created.
SQL>
SQL> insert into t values (1,2,3,4);
1 row created.
SQL>
SQL> select
2 a,
3 b,
4 c,
5 a+b-c+d/2+10-sqrt(c) as ugh
6 from t;
A B C UGH
---------- ---------- ---------- ----------
1 2 3 10.2679492
1 row selected.
SQL>
SQL> select
2 a,
3 b,
4 c,
5 ugh,
6 case when ugh > 0 then 'Y' else 'N' end more_stuff
7 from
8 (
9 select
10 a,
11 b,
12 c,
13 a+b-c+d/2+10-sqrt(c) as ugh
14 from t
15 );
A B C UGH M
---------- ---------- ---------- ---------- -
1 2 3 10.2679492 Y
1 row selected.