-Stop the OpManager service.
-Open the command prompt as Admin and navigate to OpManagerHome/bin directory.
-Run ResetPassword.bat/sh (Running this script will populate all the OpManager modules as in startup)
-It will ask confirmation to reset the admin password.
-On giving Yes, admin user password will be changed as 'admin' and it will exit.
-After resetting the password, you can start the service and use 'admin' for username and password while login.