Creating a index on a date column that's already populated Is there any downside to creating a index on a already populated date field?

Will the new index perform the same as it would if it was created when the table was? 

Would it be costly from a database perspective to add this? 

Edit : the table has just under 10 million entries.
There is no downside in creating an index at a later moment. The index will perform the same way like it would when it was created when the table was created.

You mentioned that the table has already 10 million rows. Therefore I would suggest that you create it during a 'calm' period. This because Oracle needs to lock the table. This cannot be done if there is a transaction that does an insert, update, or delete in the table.