There are many indexes for many purpose. Ie for dataware house, you need bitmap, for online you need Btree. if you are getting buffer waits in RAC due to PK index, then you can think of reverse key(other purposes are also valid).
Look for appropriate value of Initrans/maxtrans. I believe it is automatically set deponds on load, but not sure. Then you need to check tablespace block size where it is storing. You may also need to set its next incremental value. If there are many extents used by index, then consider to recreate.
Also look for clustering factor. If this value is very high, then think of re-creating it.
Next you need to find execution plan of your query.
Generate AWR reports, and thereshould not be any wait on index, then I believe you are good.
