Using CACHE option / buffer pool keep / recycle you can cache the index in memory (like we do in table).

Buffer hit ratio per table:
If you keep only one table in BUFFER POOL RECYLE , you can calculate the hit ratio by reading the columns Consistent gets, Physical reads, and db block gets from the V$BUFFERPOOL_STATISTICS table.

Other way is mapping the block numbers from V$BH to the block numbers of a table. This I did not work out. You can try.