For the RadComboBox you need to set the ObjectDataSource Id as
DataSourceID="ObjectDataSource1"
And you have to mention the Text Field and Value Field for the RadComboBox as
DataTextField="Title" DataValueField="Id"
Make the above changes and see if it works.
And also refer the Demo link below
RadComboBox ObjectDataSource[
^]
If it doesn't help please feel free to comment and also paste your full code.