The V-Ray Mtl OSL material can be used to load OSL shader code files (.osl) or OSL object code files (.oso) and render them directly with V-Ray. The V-Ray OSL Mtl plug-in can be used with shaders that write in closure color output parameters. 

These properties can also be found on the node itself.

OSL Source – Specifies whether the OSL shader code is inserted internally or loaded through a file. Use the button to manually refresh the OSL Source, in case you have entered new information into the field.

Internal – The OSL shader code is inserted inline.
External – The OSL shader code is loaded through a file. Paste the file directory into the field or click on the icon to open the File View window and select the file from there.