Naren,I can get an esri base map to overlay on top of onemap but it does not work the other way round I think it may be a projection problem, try the following code in the javascript sandbox:http://developers.arcgis.com/javascript/sandbox/sandbox.html
<!DOCTYPE html>
<html>
  <head>
    <script type='text/JavaScript' src='http://www.onemap.sg/API/JS?accessKEY=xkg8VRu6Ol+gMH+SUamkRIEB7fKzhwMvfMo/2U8UJcFhdvR4yN1GutmUIA3A6r3LDhot215OVVkZvNRzjl28TNUZgYFSswOi'></script>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no"/>
    <title>Simple Map</title>
    <link rel="stylesheet" href="http://js.arcgis.com/3.9/js/esri/css/esri.css">
    <style>
      html, body, #map {
        height: 100%;
        width: 100%;
        margin: 0;
        padding: 0;
      }
      body {
        background-color: #FFF;
        overflow: hidden;
        font-family: "Trebuchet MS";
      }
    </style>
    <script src="http://js.arcgis.com/3.9/"></script>
    <script language="JavaScript" type="text/JavaScript">
      var map;
     var centerPoint="28968.103,33560.969";
      var myLayer = new esri.layers.ArcGISDynamicMapServiceLayer("http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer");
      myLayer.opacity = 0.5;
      var OneMap = new GetOneMap('map','SM',{level:7,center:centerPoint,layer:myLayer}); //add one map here
      
      function init() {
       map=OneMap.map;
         dojo.connect(OneMap.map, "onExtentChange", showExtent);
      }
    dojo.addOnLoad(init);
    </script>
  </head>
  <body>
    <div id="map"></div>
  </body>
</html>
RegardsAnthony