How do I override a module template (.tpl.php) from my custom subtheme?
Copy file from your module folder: /var/www/openatrium/profiles/openatrium/modules/apps/oa_home/oa-home-default.tpl.php
and paste this to your subtheme in following path : /var/www/openatrium/sites/all/themes/my_radix/templates/oa-home-default.tpl.php

open this file and write something like this somewhere under php tags

 drupal_set_message('in new file');


go to admin/config/development/performance/ and clear all cache. Now when you visit to the page where the template is getting rendered, you'll see the message.