Delete all the data from all tables
Generate the statements you need

select 'DELETE FROM '||table_name||' ;' from user_tables;


Even better would be using TRUNCATE instead of DELETE.  That would prevent generating extra redo vectors in the redo log - in the end all you want to do is just get rid of the data. It would also avoid all actions by any DELETE triggers that the tables have.

select 'TRUNCATE TABLE '||table_name||' ;' from user_tables;