3 manières de faire :
SQL> flashback database to TIMESTAMP(sysdate - 2/24); --> flash back to 2 hr
SQL> FLASHBACK DATABASE TO TIMESTAMP SYSDATE-(1/24/12); --> Flashback 5 minutes.
SQL> flashback database to timestamp to_date('27-JUN-2016 19:50:00','DD-MON-YYYY HH24:MI:SS'); --> flash bakc to a specific time period
source : https://www.c-sharpcorner.com/article/flashback-database-in-oracle2/