SQL> with test as
  2    (select -20 id, 'A' val from dual union
  3     select  -5, 'B' from dual union
  4     select   7, 'C' from dual union
  5     select  16, 'D' from dual
  6    )
  7  select id, val
  8  from test
  9  where (   id < -10
 10         or id >  10
 11        );

        ID V
---------- -
       -20 A
        16 D

SQL>

SQL> WITH TEST AS
  2   (SELECT -20 ID, 10 VAL
  3      FROM DUAL
  4    UNION
  5    SELECT -15, 0
  6      FROM DUAL
  7    UNION
  8    SELECT 70, 2
  9      FROM DUAL
 10    UNION
 11    SELECT 160, -10 FROM DUAL)
 12  SELECT ID / VAL my_out
 13    FROM TEST
 14   WHERE (ID < -10 OR ID > 10);

ORA-01476: divisor is equal to zero

SQL> WITH TEST AS
  2   (SELECT -20 ID, 10 VAL
  3      FROM DUAL
  4    UNION
  5    SELECT -15, 0
  6      FROM DUAL
  7    UNION
  8    SELECT 70, 2
  9      FROM DUAL
 10    UNION
 11    SELECT 160, -10 FROM DUAL)
 12  SELECT NVL (ID / DECODE(VAL, 0, NULL, VAL), 0) my_out
 13    FROM TEST
 14   WHERE (ID < -10 OR ID > 10);
 
    MY_OUT
----------
        -2
         0
        35
       -16

NVL((EGALISATION_A1.RNI_PART - AMS.EGALISATION.RNI_PART) / DECODE(AMS.EGALISATION.RNI_PART,0, NULL, AMS.EGALISATION.RNI_PART) * 100,0)

AND (CASE
     WHEN AMS.EGALISATION.RNI_PART = 0
     THEN 0
     ELSE (EGALISATION_A1.RNI_PART - AMS.EGALISATION.RNI_PART) / AMS.EGALISATION.RNI_PART * 100
     END > 10
     OR
     CASE
     WHEN AMS.EGALISATION.RNI_PART = 0
     THEN 0
     ELSE (EGALISATION_A1.RNI_PART - AMS.EGALISATION.RNI_PART) / AMS.EGALISATION.RNI_PART * 100
     END < -10
     )

SQL> WITH TEST AS
  2   (SELECT -20 ID, 10 VAL
  3      FROM DUAL
  4    UNION
  5    SELECT -15, 0
  6      FROM DUAL
  7    UNION
  8    SELECT 5, 2
  9      FROM DUAL
 10    UNION
 11    SELECT 160, -10 FROM DUAL)
 12  SELECT ID, VAL, NVL(ID / DECODE(VAL, 0, NULL, VAL), 0) MY_OUT
 13    FROM TEST
 14   WHERE
 15  -- (ID < -10 OR ID > 10)
 16  (NVL(ID / DECODE(VAL, 0, NULL, VAL), 0) < -10 OR NVL(ID / DECODE(VAL, 0, NULL, VAL), 0) > 10);
 
        ID        VAL     MY_OUT
---------- ---------- ----------
       160        -10        -16

SELECT egl.nptf AS fund, --the ALL is unneccessary
      egl.tparts AS shareclass,
      eg2.date_nav AS "Current Day",
      eg2.rni_part AS "Current Income",
      egl.date_nav AS "Previous Day",
      egl.rni_part AS "Previous Income",
      CASE WHEN egl.rni_part = 0
           THEN 0
           ELSE (eg2.rni_part - egl.rni_part) / egl.rni_part * 100
      END AS "% Movement"
FROM ams.egalisation  egl, --alias for column refs, makes the whole thing easier to read
     ams.egalisation egl2 --shorter, quicker to type, easier to read
WHERE egl.nptf = eg2.nptf
  AND egl.tparts = eg2.tparts
  AND egl.cega = eg2.cega
  AND ABS(CASE WHEN egl.rni_part = 0
           THEN 0
           ELSE (eg2.rni_part - egl.rni_part) / egl.rni_part * 100
       END) > 10
  AND (egl.nptf LIKE 'S2%'  
   OR egl.nptf LIKE 'S1%')
--  AND (egl.nptf NOT LIKE 'S25%'))) --If either of the above are true, then this cannot be true and is therefore unneccessary
  AND egl.date_nav = trunc(SYSDATE) - 4
  AND eg2.date_nav = trunc(SYSDATE) - 1
  AND egl.cega = 'GB'
  --Removed a lot of unnecessary parens and fixed the ones around the OR 
ORDER BY egl.nptf;