I don't know if Oman has any special settings.
But generally speaking there isn't much to do.
Are you sure it has the right password?
Try connecting to another account.
You also might try downloading AppDelete and removing MSN Messenger thru that (or a similar program).
Then go the Keychain and make sure nothing for MSN messenger remains.
Than try to reinstall it.
But, I really doubt your problem is with the program itself.
Either some settings are incorrect and it can't connect, or the username and password are incorrect.
It is not uncommon for MSN to respond with invalid user/password, when in reality it can't connect itself.
Also, check the firewall under System Preferences> Security, and make sure that MSN has the right authorization and open ports.