ServiceLocation__1_0.php

    $public_fields['organization'] = array(
      'callback' => function (DataInterpreterInterface $data) {
        return $data->getWrapper()->og_group_ref->label();
      }
    );

    $public_fields['organizationLogo'] = array(
      'callback' => function (DataInterpreterInterface $data) {
        $style = 'small_logo';
        $logo_field = $data->getWrapper()->og_group_ref->field_logo->value();
        $url = image_style_url($style, $logo_field['uri']);
        return $url;
      }
    );