SQL> create table log_sal
  2    (empno     number,
  3     old_sal   number,
  4     new_sal   number
  5    );

Table created.

SQL> create or replace trigger trg_bu_sal
  2    before update of sal on emp
  3    for each row
  4  begin
  5    insert into log_sal
  6      (empno, old_sal, new_sal)
  7      values
  8     (:old.empno, :old.sal, :new.sal);
  9  end;
 10  /

Trigger created.

SQL> select empno, sal from emp where ename = 'SCOTT';

     EMPNO        SAL
---------- ----------
      7788       3800

SQL> update emp set sal = 38 where ename = 'SCOTT';

1 row updated.

SQL> select empno, sal from emp where ename = 'SCOTT';

     EMPNO        SAL
---------- ----------
      7788         38

SQL> select * from log_sal;

     EMPNO    OLD_SAL    NEW_SAL
---------- ---------- ----------
      7788       3800         38

SQL>