OS 390 is an IBM operating system so there is a very good chance that the database is DB2 or even maybe Informix.

Also, OS 390 is IBM’s platform for running Lotus Notes/Domino, a mail/document handling system that thinks it’s a dbms.

OS390, now Z/OS, supports DB2, and IMS, both IBM Database systems. Other vendor DB’s include Oracle, Informix, IDMS, M204, Tameno(Software AG XML database), and probably some others. Remember Z/OS which started out life as MVT, then MVS, OS390, and now Z/OS have been running Database systems since the late 60′s.

OS/390, now z/OS can run any of number of DBMS systems. We’re running zOS 1.3 with our criminal justice system utilizing CA’s Datacom/DB database (old, costly and non-relational even though it can handle SQL) and financials on DB/2. Oracle and Informix also run on z/OS. Another DBMS you may want to consider is Model 204 which hardly anyone hears about because they (Computer Corporation of America) don’t advertise. It’s supposed to be one of the best performing RDBMS systems available. The New York public school system runs Model 204 with over 1 million students.

I don’t personally know anything about Lotus Notes, but it appears that you would be able to access it via Websphere.
The following is a quote from IBM’s Domino site”

“IBM is including a limited license for WebSphere Application Server for use with Domino applications, to enable the use of servlets, Java Serverpages (JSPs) and Web services. The WebSphere code will automatically be available to Domino purchasers from their personalized Passport Advantage site. Special license terms allow WebSphere use on the same physical machine as Domino, accessing Domino objects and data (no Java connector or external data access permitted).”

Hopefully this gives you something to look into further.