We have been asked a number of times if it is possible for users/members of an OptimizeMember site to login using their email address and not their username.
Optimizemember does not provide this functionality as standard, and we have no plans to currently. Changing the username format/layout is a core function of WordPress and could potentially cause issues in the future.
However, if you really need this functionality, there is a plugin which allows this functionality:
You can install this on your site by going to Plugins > Add New and search for WP Email Login
You should be ok adding this plugin, but you’ll need to keep an eye out for incompatibility in the future. As long as it’s just allowing Users to log in with their email addresses, you should be able to disable the plugin if support for it drops.