This doesn't separately discuss online-reorg, but rather uses various options of REORG control statement to tell you REORG as a complete utility.
The obvious advantage is that table can be accessed most of the time while the reorg is running. And if the utility fails due to some reason, the tablespace won't need to be recovered. So it is clean and cost of it is same as offline reorg.
I suppose disadvantage would be that you will require additional space amounting to the same that the original tablespace occupies. And there is a mapping table to be created.
There are several more restrictions and dependencies. See the manual above for more.