After creating a data source using webapi\odata, I realized that mapping an optionset required an Enum as per Microsoft and OData requirements
 
See my example below: Creating the OData one can define these Enums:
 
 
event type code example

Quering the $metadata I would see the enums defined as follow:

meta data enum returned

mappng these is then as ealy as :

mapping
  1. *** note  The ‘External Type name’ : The actual ‘type name’.’name’ (this got me stumped for a while no-where was this documented)
  2. Member Name.