We were having high ITL waits and high number of deadlocks (due to the hanging transactions waiting on the ITL on the data block) on a table and index and part of the fix was to increase the INITRANS on the table and on the index.
INITRANS is a physical attribute that determines the initial number of concurrent transaction entries allocated within each data block for a given table/index/cluster. Every transaction that updates a block has to acquire an Interested Transaction List (ITL) slot, in order to store the transaction id, rollback information and ultimately lock the necessary row/index entry within the block.
For an existing object it needs to be rebuild as INITRANS is a physical attribute on the Oracle datablock. So if you change on a table and you want it to take effect for the current data you need to move the table or one partition for example.
If you are doing this type of change on a partitioned index you need to change the index default attribute as well rebuild the index partition or the entire index.
But one thing caught my eye while working on this as after setting the new INITRANS default attribute and rebuilt the index online it did not changed the INITRANS values.
See below the execution on a test system