Initialize those Settings

carrierwave.rb

CarrierWave.configure do |config|
  config.fog_credentials = {
    provider: 'AWS',
    aws_access_key_id: Settings.aws.access_key,
    aws_secret_access_key: Settings.aws.secret
  }
  config.fog_directory = Settings.aws.bucket
end

settings.rb

class Settings < Settingslogic
  source "#{Rails.root}/config/settings.yml"
  namespace Rails.env
end

settings.yml

defaults: &defaults
  use_online_storage?: false
  aws:
    access_key: longstringkey
    secret: supersecret
    bucket: some-bucket-name


development:
  <<: *defaults

test:
  <<: *defaults

production:
  <<: *defaults
  use_online_storage?: true