In the Oracle Server, what are the things that will happen when we commit a transaction? Can we still undo what we have committed already?

When you update the Oracle Server in some way, that update should initially be regarded as tentative only - tentative in the sense that, if something subsequently goes wrong, the update may be undone. Updates remain tentative until an SQL COMMIT or SQL ROLLBACK statement is executed. The SQL COMMIT statement makes all tentative updates firm or committed. Once committed, an update is guaranteed never to be undone.

Yes you are right. Thanks for your update.I even tried a example for my understanding.