title: What are oaidl.idl and ocidl.idl for?
body: <p>A default ATL Simple Object has the following at the top of its IDL file:</p>
  
  <pre><code>import "oaidl.idl";
  import "ocidl.idl";
  </code></pre>
  
  <p>What are these files for, and how do I know when I need to import them? Is there any documentation which describes what is defined in these two files? I have no idea what "oa" and "oc" stand for, and there are no helpful comments in the files themselves.</p>