An Oracle index-organized Table is a table stored in an index B-Tree
structure. There is no second data structure (heap-table) for the table. The Oracle
database always uses the primary key as clustering key. An Index-Organized
Table is created with the
ORGANIZATION INDEX clause:
CREATE TABLE ( id NUMBER NOT NULL PRIMARY KEY, [...] ) ORGANIZATION INDEX
SQL Server supports index-organized tables as well, but uses the term clustered index.