Working with OMF Tablespaces

Using OMF (Oracle Managed File), you don’t explicitly name datafiles or temp files; the database does this for you. To enable OMF, set the initialization parameter DB_CREATE_FILE_DEST to the directory where you want the database to create and manage your data and temp files.

ALTER SYSTEM SET
db_create_file_dest=’D:\sample\OMF’
SCOPE=BOTH;

When creating a tablespace with OMF, you simply omit the filename:
CREATE TABLESPACE ; [OR] CREATE BIGFILE TABLESPACE ;

By default the datafile created using OMF will have auto extend enabled and be 100MB unless you specify a different size.

Reference: Sybex Oracle 10g Admin I Study Guide