<script type="text/javascript">
    function onoff(){
        currentvalue = document.getElementById('onoff').value;
        if(currentvalue === "Off"){
            document.getElementById("onoff").value="On";
        }else{
            document.getElementById("onoff").value="Off";
        }
    }

</script>

<form action="" method="post">
<input type="button"  name="OnOff" value="Off" id="onoff" onclick="onoff();" class="button button2">
</form>

طبعا المفترض يحول بين On و Off لكن بعد وضعه داخل tag form ما يتغير صار ثابت على Off