Indexes are old friends of every DBA. To check out what your friends are up to, in Oracle, basic way is enable monitoring for the index by:

Then you can query V$OBJECT_USAGE to see if it is being used. After a period of time meaningful to you, you can stop monitoring till the next time by:

However what is lacking in this way of monitoring is statistics. You can see whether index is used but cannot see how many times or how it is used. Fortunately Oracle provides many V$ views. The query below returns basic statistics about your indexes. How it is used (range or full scan etc.), how many times and when was the last time it was used:

select sp.object_name as index_name,
sp.options,
sum(sa.executions) as executions#,
max(timestamp) as last_used
from v$sql_plan sp, v$sqlarea sa
where sa.address = sp.address
and sa.hash_value =sp.hash_value
and sp. operation = 'INDEX'
and object_owner in ('SCOTT')
group by object_name, options
order by 3 desc;

With Exadata, we've started to use less indexes and even dropped some existing ones. Because we want to use Exadata features like storage indexes and smart scan instead, we let the cells do their job. On Exadata we prefer to see fast full scans instead of range scans. To see Exadata specific statistics related to indexes, you can add columns with prefix "io_cell" in V$SQLSTATS or V$SQLAREA views. Such as:

select sp.object_name as index_name,
sp.options,
sum(sa.executions) as executions#,
max(timestamp) as last_used,
sum(sa.disk_reads) as disk_reads,
sum(sa.direct_writes) as direct_writes,
sum(sa.io_cell_offload_eligible_bytes)/1024/1024 as offloadable_mb,
sum(sa.io_cell_offload_returned_bytes)/1024/1024 as offloaded_mb,
sum(sa.rows_processed) as rows_processed
from v$sql_plan sp, v$sqlarea sa
where sa.address = sp.address
and sa.hash_value =sp.hash_value
and sp. operation = 'INDEX'
and object_owner in ('SCOTT')
group by object_name, options
order by 3 desc;

There are many other statistics you can find in V$ views, so you can improve the query according to your needs.