I'm also looking into this at the moment. In fact posted here but no answers. To the best of my limited knowledge, it would seem that to call a stored procedure on the mainframe from Oracle, you would need additional software ibn the form of Oracle Transparent Gateway for IBM DRDA .
There is an alternative way to connect to DB2 from Oracle called Generic Connectivity which uses ODBC. This is part of the base product so no extra cost, but it does not allow you to call stored procs.