You can use Open as well as Classic Agents in the same Automation Project. Such an environment is called as "Mixed Agent Environment".
And Open Agent directly supports Dynamic Object recognition. The latest SilkTest 2009 has new ways of implementing the Dynamic Object Recognition (DOR) using the "locator" keyword. It is much similar to that of Classic Agent "tag" keyword.
However, ST 2008 does not support locator keyword, it has a different clumsy approach.
You can use dynamic object recognition with classic agent
you will use identifier for the form where the object is located, followed by the class of the object, followed by the tag, followed by the action
<font class="small">Code:</font><hr /><pre>MyApp.MyForm.ComboBox("City").Select("#2") </pre><hr />