Though OpenLayers does define an opacity property for the layer object, I haven’t been able to get it working. Therefore, I have had to direct define opacity within the SLD associated with the layer I am interested in making transparent. If you have defined the Polygon SLD for your feature type: within the administration web interface, navigate to Config>Data>Style> choose the Polygon SLD and edit this. Finally find the tag. Within this you should find a tag wtih a corresponding value. You need to create a new tag value pair that should look something like this 0.7. This parameter tells the map viewer to display this layer with an opacity of 70%.