Sample: Extending ArcGIS Server JS API TiledMapServiceLayer to add OpenStreetMap Layer

In this sample the OpenStreetMap Layer is added as basemap and World population layer which is a dynamic map service hosted on ArcGISOnline sample server is added as an overlay. Use the transparency slider to control the opacity of the overlay.Refer to this link for documentation on creating custom layer types

Change World Population layer transparency (0.3)