Hi Vincent,
You are specifying the baseLayer as a string rather than the variable you defined previously. You will also need to add the 'visible' option and set this to true. Ex:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<!--The viewport meta tag is used to improve the presentation and behavior of the samples
on iOS devices-->
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no">
<title>Overview Map</title>
<link rel="stylesheet" href="http://js.arcgis.com/3.14/dijit/themes/claro/claro.css">
<link rel="stylesheet" href="http://js.arcgis.com/3.14/esri/css/esri.css">
<style>
html, body { height: 100%; width: 100%; margin: 0; padding: 0; }
</style>
<script src="http://js.arcgis.com/3.14/"></script>
<script>
var map;
require([
"esri/map", "esri/dijit/OverviewMap", "esri/layers/ArcGISDynamicMapServiceLayer",
"dojo/parser",
"dijit/layout/BorderContainer", "dijit/layout/ContentPane", "dojo/domReady!"
], function (
Map, OverviewMap, ArcGISDynamicMapServiceLayer,
parser
) {
parser.parse();
map = new Map("map", {
basemap: "streets",
center: [-122.445, 37.752],
zoom: 14
});
var BM_OM = new ArcGISDynamicMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer",{
useMapImage: true
});
BM_OM.on("load", function(evt){
var overviewMapDijit = new OverviewMap({
map: map,
baseLayer: BM_OM,
visible: true
});
overviewMapDijit.startup();
});
});
</script>
</head>
<body class="claro">
<div data-dojo-type="dijit/layout/BorderContainer"
data-dojo-props="design:'headline', gutters:false"
style="width: 100%; height: 100%; margin:0;">
<div id="map"
data-dojo-type="dijit/layout/ContentPane"
data-dojo-props="region:'center'"
style="padding:0">
</div>
</div>
</body>
</html>
