In http://docs.mongodb.org/manual/tutorial/backup-with-mongodump/#point-in-time-operation-using-oplogs, we refer to Point In Time operation.
This is confusing, as a reader may confuse the concept with real Point In Time restores.

Using the --oplog option allows to perform a backup on a live database that is coherent once restored, it does not allow you to restore to a specific point in time.