The OSM Mapnik tiles from http://a.tile.openstreetmap.org/ are fully opaque PNG tiles which makes it a base layer and unusable as an overlay.
Most WWJ applications would likely prefer to use OSM as an overlay.
The NASA OpenStreetMap map cache at https://worldwind20.arc.nasa.gov/mapcache served OSM overlay tiles that included transparent voids, however that server is currently returning 404 Not Found for /mapcache
Using OSMMapnickLayer with a default 50% opacity is a simple substitution for the broken NASA OSM layer.