If you set TOracleSession.ThreadSafe to True for all sessions, you can still use a Timer to call TOracleSession.CheckConnection(True) for each session. This will be blocked by simultaneous access from the threads, but it will work okay.

The CheckConnection function will keep the session alive by preventing too much idle time, and will (attempt to) reconnect in case of fatal other failures, such as a killed session, a database that was shutdown and restarted, network failures, and so on.