The DisplayMemberPath property is not respected

Workaround:
Use an implicit style targeting RadOutlookBarItem instead of the DisplayMemberPath property. You can define the style in the xaml page (the view) where the RadOutlookBar control is defined.

<Window.Resources>
    <Style TargetType="telerik:RadOutlookBarItem">
        <Setter Property="Header" Value="{Binding ItemHeader}" />
    </Style>
</Window.Resources>