First tell Oracle about your DLL...
Then create a 'wrapper' function for each function within the DLL.
CREATE OR REPLACE LIBRARY MY_DLL_LIB AS 'C:\ORACLE\ORA81\MY_DLL.DLL';
You will have to ensure that EXTPROC is correctly configured, and of course write the DLL. You can then call the DLL function as if it were a PL/SQL function...
FUNCTION TestFun RETURN VARCHAR2 IS
EXTERNAL LIBRARY MY_DLL_LIB
NAME "DllTestFun" -- this is the dll function
Check otn.oracle.com for calling conventions etc.
Result := MY_DLL_LIB.TestFun;
Please don't email me directly with questions. I've probably just got home from the pub and cannot guarantee the sanity of my answers. In fact, I can't believe I actually made it home.