The NOLOGGING clause doesn’t prevent redo on all operations, but rather only on a subset. I searched the documentation for examples of this…
The [NO]LOGGING clause applies to tables, partitions, tablespaces, and indexes. Virtually no log is generated for certain operations (such as direct-path INSERT) if the NOLOGGING clause is used. The NOLOGGING attribute is not specified at the INSERT statement level but is
instead specified when using the ALTER or CREATE statement for a table, partition, index, or tablespace.
When a table or index has NOLOGGING set, neither parallel nor serial direct-path INSERT operations generate redo logs. Processes running with the NOLOGGING option set run faster because no redo is generated. However, after a NOLOGGING operation against a table,
partition, or index, if a media failure occurs before a backup is performed, then all tables, partitions, and indexes that have been modified might be corrupted.
Direct-path INSERT operations (except for dictionary updates) never generate redo logs if the NOLOGGING clause is used. The NOLOGGING attribute does not affect undo, only redo. To be precise, NOLOGGING allows the direct-path INSERT operation to generate a negligible
amount of redo (range-invalidation redo, as opposed to full image redo).
But I did find an Ask Tom article which is more explicit about what operations generate redo despite the NOLOGGING clause:
This has a nice table of operations with either No/Archive and No/Logging specified, and you can see redo is generated most of the time.
So to sum up the NOLOGGING clause only works with certain operations and we cannot expect all REDO to be completely halted.
Another item to consider is whether the Indexes on the tables were created with NOLOGGING or not… This is covered in Index generates high redo, although it is in NOLOGGING (Doc ID 1235234.1), so please reveiw that note to see if there are some indexes that can be recreated to reduce redo further.