SELECT COUNT (A.ID) SEARCHED_MASTERLS
  FROM CV_MASTERL A
 WHERE 1 = 1
   AND (    1 = 1
        AND (ID IN (
                SELECT /*+ INDEX(B) */
                       A.ID
                  FROM MASTERL A, MARKETING B
                 WHERE (B.FF_ID = '1' OR B.FF_ID IS NULL)
                   AND A.ID = B.MASTERL_ID(+)
                   AND NVL (UPPER (B.VALUE), '¤') LIKE '%PARTNER%')

            )
       )

Table structure
CREATE TABLE MASTERL
(
  ID                 NUMBER(12)                 NOT NULL primary key,
  DATABASE_ID        NUMBER(12)                 NOT NULL,
  MASTERLNO             VARCHAR2(15 BYTE),
  FIRSTNAME          VARCHAR2(255 BYTE)
)

CREATE INDEX MASTERL_DATABASE_FK_I ON MASTERL (DATABASE_ID)

CREATE TABLE MARKETING
(
  DB_ID         NUMBER(12)                      NOT NULL,
  FF_ID         NUMBER(12)                      NOT NULL,
  MASTERL_ID       NUMBER(12)                      NOT NULL,
  VALUE         VARCHAR2(255 BYTE)
)
CREATE INDEX MARKETING_DB_ID_IDX ON MARKETING (DB_ID)
CREATE INDEX MARKETING_IDX ON MARKETING (MASTERL_ID, DB_ID)

Operation	 	   	   	   		  			 	  	   		 	OBJECT NAME		  				ROWS 	Bytes	   COST		   OBJECT Node		IN/OUT		PStart		PStop

-SELECT STATEMENT Optimizer MODE=ALL_ROWS		  		 				   			  			  			1  		   14 M	 	      	             	 
	  -SORT 	  																			1  	58  	 	 	      	             	 
	   	-HASH JOIN RIGHT OUTER																		124 K	6 M	   14 M	 	      	             	 
		  	  INDEX FAST FULL SCAN									CALC.CAMPAIGN_PK						20 K	99 K	   15  	 	      	             	 
			-HASH JOIN SEMI																		124 K	6 M	   14 M	 	      	             	 
			 	 -TABLE ACCESS BY INDEX ROWID							CALC.MASTERL							7 K	90 K	   2167  	 	      	             	 
				 	-NESTED OOPS																527 K	20 M	   31967  	 	      	             	 
						-NESTED LOOPS															69  	1 K	   22  	 	      	             	 
							-TABLE ACCESS BY INDEX ROWID				CALC.USERS							1  	15  	   2  	 	      	             	 
								   INDEX RANGE SCAN				CALC.USERS_DBUSERNAME_I						1  	  	   1  	 	      	             	 
							-TABLE ACCESS BY INDEX ROWID				CALC.DATABASE							69  	897  	   20  	 	      	             	 
								   INDEX RANGE SCAN				CALC.DATABASE_CLIENT_FK_I					43  	 	   1  	 	      	             	 
						INDEX RANGE SCAN	   					CALC.MASTERL_DATABASE_FK_I					60 K	 	   213  	 	      	             	 
				  -VIEW	SYS.VW_NSO_1															   	616 M	7G	   14 M	 	      	             	 
				  	-FILTER		  	 	 	 	      	             	 
						-HASH JOIN OUTER								 						616 M	12G	   14 M	 	      	             	 
							INDEX FAST FULL SCAN					CALC.MASTERLS_PK						222 M	1G	   167570  	 	      	             	 
						  -TABLE ACCESS BY INDEX ROWID					CALC.MARKETING							600 M	7G	   12 M	 	      	             	 
						    INDEX FULL SCAN		 				CALC.FORMFETLVALUS_DB_ID_IDX					600 M	 	   2067451  	 	      	             	 
	   	   

SELECT A.ID
FROM MASTERL A, MARKETING B
WHERE (B.FF_ID = '1' OR B.FF_ID IS NULL)
AND A.ID = B.MASTERL_ID(+)
AND NVL (UPPER (B.VALUE), '¤') LIKE '%PARTNER%'  ---This condition causes slowdown. How can this be improved?

SELECT COUNT (A.ID) SEARCHED_MASTERLS
FROM CV_MASTERL A
INNER JOIN MASTERL A1 on A.ID=A1.ID
LEFT JOIN MARKETING B ON A1.ID = B.MASTERL_ID
                         AND NVL (UPPER (B.VALUE), '¤') LIKE '%PARTNER%')
WHERE  NVL(B.FF_ID,'1') = '1'

SELECT A.ID
FROM MASTERL A
LEFT JOIN (SELECT B.MASTERL_ID, B.FF_ID 
           FROM  MARKETING B
           WHERE NVL(B.FF_ID,'1') = '1'
                AND  UPPER (B.VALUE) LIKE '%PARTNER%') C ON C.MASTERL_ID=A.ID
WHERE NVL(C.FF_ID,'1') = '1'

SELECT A.ID
FROM MASTERL A, MARKETING B
WHERE (B.FF_ID = '1' OR B.FF_ID IS NULL)
AND A.ID = B.MASTERL_ID(+)
AND NVL (UPPER (B.VALUE), '¤') LIKE '%PARTNER%'

SELECT COUNT (A.ID) SEARCHED_MASTERLS
  FROM CV_MASTERL A
 WHERE ID IN (SELECT B.ID
              FROM MASTERL B)

(B.FF_ID = '1' OR B.FF_ID IS NULL)

B.FF_ID(+) = '1'

AND NVL (UPPER (B.VALUE), '¤') LIKE '%PARTNER%'

SELECT COUNT (A.ID) SEARCHED_MASTERLS
  FROM CV_MASTERL A
 WHERE ID IN (SELECT A.ID
              FROM   MASTERL A, MARKETING B
              WHERE  B.FF_ID = '1'
              AND    A.ID = B.MASTERL_ID)
              AND    UPPER (B.VALUE) LIKE '%PARTNER%')

SELECT COUNT (A.ID) SEARCHED_MASTERLS
  FROM CV_MASTERL A
 WHERE ID IN (SELECT A.ID
              FROM   MASTERL A, MARKETING B
              WHERE  B.FF_ID = '1'
              AND    A.ID = B.MASTERL_ID)
              AND    INSTR(B.VALUE,'PARTNER')>0

The lines (B.FF_ID = '1' OR B.FF_ID IS NULL)is a poor mans version of the oracle outer join notation.
The Outer Join record returned if there is no match consists entirely of null values, so these clauses are just taking account of that possibility.

It is syntactically the same, but less clear thanB.FF_ID(+) = '1'