Update Query said "ORDER_DETAILS"."QUANTITY": invalid identifier
For reference, this is a continuation of the following post.
How to convert an Update a query from MS Access to Oracle SQL Software

I kind of wish you would have continued our conversation there, but I guess they can be merged somehow.

I think you might have taken what I said literally and just switched ORDER DETAILS table to ORDER_DETAILS in your SELECT statement instead of changing the actual object's name. 

The object's name was [ORDER DETAILS] and needs to be identified as "ORDER DETAILS" in ORACLE (with Quotes instead of brackets).  Try that and see.

If not, and you actually did change the table itself to ORDER_DETAILS, then your issue is that you're using ORDER_DETAILS.Quantity in your SET.  That is not appropriate.  You NEED to give the subquery that the UPDATE wraps an ALIAS and say:

SET Alias.OldValue = Alias.NewValue


You should set up your SELECT to retrieve the OldValue of the column, and the NewValue of the column.  Please see the link I included in my original answer:
https://stackoverflow.com/questions/2446764/update-statement-with-inner-join-on-oracle