Adding an OpenStreet baseMap

Hey everybody.

I work with GeoServer using Apache Tomcat in NetBeans. What I want is using OpenStreetMap as base map of my data. So I added this code:

layer = new OpenLayers.Layers.OSM(); map.addLayer([layer, layer1, layer2]);

before adding this code everything works perfectly, but now when I try to execute my HTML page, fireBug shows the folowing error

TypeError: OpenLayers.Layer.OSM is not a constructor. osm = new OpenLayers.Layer.OSM();

Thanks for your collaboration :)