Getting other layers to play nicely with these layers involve three things. These rules are also available for others projections:
Set up the correct map projection properties, in this case, the default is already the right projection.
Make sure that projection layer is set to the right EPSG code (most tiles' layers also use the right projection) and are available on the third-party map layer.
Ensure all raster layers (any non-Vector or Image layer), such as WMS, are in the map's projection. In this case, we'll need to make sure we ask our WMS server for map tiles in the EPSG:3857 projection.
Using what we've learned so far, let's make a mashup. We'll use OSM derived layers with Bing Maps layer and put at the top a WMS layer. We will also set a small layer switcher (as there is no available component in OpenLayers core code for this).