To enable 'LoadOndemand' feature for a node, the node property, 'ExpandMode' should be set as 'ServerSideCallback'

    <obout:Tree ID="ObClassicTree" ClientObjectID="obTreeClientObj" DataSourceID="XmlDataSource1"
        EnableViewState="false" LoadingMessage="Loading..." CssClass="vista" runat="server" Width="200px">
        <DataBindings>
            <obout:NodeBinding DataMember="employee" ExpandMode="ServerSideCallback" TextField="name"
                ImageUrl="~/obout.ajax.ui/treeview/examples/img/engineer-icon.png" />
        </DataBindings>
    </obout:Tree>
            

            function onCheckChange(obj) {
                //This property can also be set in server side
                obTreeClientObj.loadingMessage = obj.checked ? 'Refreshing...' : 'Loading...';
            }