How do the users login to legacy system?
How does the legacy system login to the database?

The most common scenario is:
a) Your OID is synchronized with M$ Active Directory where user accounts and passwords are managed.
b) Applications use LDAP to validate login.

Therefore, you may need to change the legacy login procedure to use LDAP for account validation.

PS: If you are lucky, the legacy application may already have LDAP configured and you would just have to "turn it on".