This option is really very useful when you have database for 24×7.
alter index in_tab_01 rebuild online;
The clause rebuilds the index without affecting the DML accessing it.
It does so by tracking which blocks are being accessed and at the end merging these blocks with the newly built index. To accomplish this task the operation had to get an exclusive lock at the end of the process. Although short in duration, it was a lock nevertheless, and DMLs had to wait.
In Oracle Database 11g, the online rebuild is truly online: it does not hold an exclusive lock. The DMLs are not affected.
Thanks to Oracle Technet