When you open "Monitoring -> Graphs" You get SQL errors rather than graphs:

ociexecute(): ORA-00907: missing right parenthesis [include/db.inc.php:461]
SQL error [ORA-00907: missing right parenthesis] in [SELECT DISTINCT g.* FROM groups g,hosts_groups hg,hosts h WHERE ((g.groupid BETWEEN 000000000000000 AND 099999999999999)) AND hg.groupid=g.groupid AND h.hostid=hg.hostid AND h.status=0 AND EXISTS (SELECT 1 FROM items i,graphs_items gi WHERE i.hostid=hg.hostid AND i.itemid=gi.itemid ROWNUM <=1)]
ocifetchinto(): ORA-24374: define not done before fetch or execute and fetch [include/db.inc.php:625]

Using Oracle as backend: SQL errors
Using MySQL as backend: all is normal