This happens if you're using the Compact Material Editor and the Channel Name in the Map Output Selector isn't set. So all you need to do is set that to Col and the material will now show in the material editor.
The Map Output Selector gets automatically added between the map slot and the OSL Map when you create it. For background see the 2nd note on the Open Shading Language (OSL) page in the online help.
In case you use the Slate Material Editor the Map Output Selector is hidden and the different outputs are shown as outputs
If you change between different OSL files which don't provide the same outputs sometimes the material editor gets confused, the quickest way to resolve that is to manually change the Channel Name (Compact Material Editor) or reconnect the OSL output to the material input.
If youn use the Arnold renderer make sure to use an Arnold Standard Surface instead of a Standard material!
In the first sample slot the Channel Name is not set and therefore it doesn't show the map:
The second sample slot has the Channel Name set to Col and all is fine: