I don't have OE set up where I am at the moment, so I'm going from memory which may not be exactly correct....
In OE, File, Manage Identities. Create your new Identities like any normal setup (with the appropriate name, mail account info, etc.). Then back at the main 'manage identities' window, (this part varies by OE version) make sure you're set to not have a default identity for any programs. That way, the user will always be prompted for the correct identity at OE starup.
Here's the hard part....train your users to always select File, Logoff identity when exiting OE. With the 'always ask condition' mentioned above, this shouldn't be necessary, but it makes OE behave more reliably.