reorg rebuild ... online includes these restrictions:
Tables on which you run reorg rebuild ... online must have a unique index.
All DMLs—that is, select (but not select into), insert, update, and delete—can operate on a table while reorg rebuild ... with online is running. Adaptive Server does not allow inserts that lead to page splits on tables with all-pages locking scheme while reorg rebuild ... online is running.
You cannot run more than one instance of reorg rebuild ... online simultaneously on a table.
reorg rebuild ... online does not materialize non-nullable nonmaterialized default columns.