The columns will be created by default nullable and this will be very fast. Note, that Oracle still will nees an exclusive lock on the table for a very short moment (like a second). After that you can start populating columns.
What you should take care of is that you don't cause a situation with many migrated rows when populating these columns later.
Does the following SQL touch every single row immediately?
ALTER TABLE supplier ADD (supplier_tax_id varchar2(10), supplier_timezone varchar2(5) );
-- Best regards, Alex Gorbachev
http://blog.oracloid.com -- //www.freelists.org/webpage/oracle-l