Convert SQL Server UPDATE Statement for Use in Oracle
Just remove the FROM clause and replace the reference to the alias with the table name:

UPDATE 
    SOB
SET 
    COA =   CASE 
                WHEN    
                    (
                        SELECT COUNT(*)
                        FROM SOB s2
                        WHERE
                            s2.ORD = SOB.ORD AND 
                            s2.SHP_KEY &lt;= SOB.SHP_KEY AND 
                            s2.SHP_DTTM &lt;= SOB.SHP_DTTM AND 
                            s2.SOB_KEY &lt;= SOB.SOB_KEY
                    ) &gt; 1 THEN 0 
                ELSE 1 
            END
WHERE 
    COA IS NULL