I've actually managed to upgrade OMG to Rails 5 without much of a problem. Surprise, surprise. Although, the only issue I encountered was with Devise parameter sanitizer. There was a change in the syntax that got me dreading for a while (the typical response after seeing an error page) but was quickly fixed after a quick Google search as usual.

Stack Overflow saves the day once again.

def configure_permitted_parameters
  devise_parameter_sanitizer.permit(:sign_up, keys: [:username])
end