The SELECT inside the OPENQUERY INSERT/SELECT can only reference field names, if you use SELECT TO_CHAR(..) then that column is a function result and not a table column that can be written to. The function value columns will be read-only no matter what permissions you have on the table. But that's not your only problem.

SQL Server timestamp columns are not datetime values, they are just integers. (Actually they are BINARY(8), but you can convert them to bigints). Every record that is inserted or updated in table with a timestamp column gets the next sequential value. It's like an IDENTITY field, but there is only one sequence per database. There is no way to turn this back into a datetime. You can compare two records and know which one is more recent, but you don't know when it happened.

If you need an actual date&time value that can be exported to Oracle you probably need an INSTEAD OF INSERT,UPDATE trigger that sets a datetime field to GETDATE() (or CURRENT_TIMESTAMP if you want to be less SQL Server-centric).