blocks that reside in the buffer_cache

select o.object_name,count(*) number_of_blocks
from dba_objects o, V$BH bh
where o.data_object_id=bh.objd
and o.owner!='SYS'
group by o.object_name
order by count(*) desc



Hit ratio:
select name,physical_reads,db_block_gets,consistent_gets,1-(physical_reads/(db_block_gets+consistent_gets))"HIT RATIO" from v$buffer_pool_statistics