<obout:Combobox id="cbo1" runat="server"
     OnClientChange="jsChange()" OnClientOpen="onClientOpen" OnClientClose="onClientClose">
    <Options>
        <obout:Option Value="0" runat="server">asptreeview</obout:Option>
        <obout:Option Value="1" runat="server">calendar</obout:Option>
        <obout:Option Value="2" runat="server">combobox</obout:Option>
    </Options>
</obout:Combobox>

<script type="text/javascript">    
    function jsChange() {
        alert("The Text of the combobox was changed to: '" + cbo1.getText()
             + "' and the Value to: '" + cbo1.getValue() + "'");
    }  
    function onClientOpen() {    
        alert("The combobox was opened.");
    }
    function onClientClose() {
        alert("The combobox was closed.");
    }  
</script>